Dentifrice-compatible silica particulates

ABSTRACT

Novel silica particulates especially adapted for formulation to dentifrice compositions exhibit unique physical and chemical properties. In one embodiment, silica particulates have a unique surface chemistry as to be at least 50% compatible with zinc values, and have a number of OH functions, expressed as OH/nm 2 , of at most 15 and a zero charge point (PZC) of from 3 to 6.5. In a second embodiment, particulates have a surface chemistry as to be at least 65%, and preferably at least 90% compatible with guanidine values, notably chlorhexidine, and acidity function thereof, Ho, of at least 3.3. In a third embodiment, silica particulates are compatible with organic amines, and have a pH, in aqueous suspension, which varies according to the equations pH≦7.5-0.7 log(C) and pH≧5.0-0.5 log(C) and which also varies as a function of the electrical conductivity thereof, according to the equations pH≦8.5-0.4 log(D) and pH≧7.0-0.6 log(D) wherein (C) represents the weight concentration of said silica suspension, expressed % SiO 2  and (D) represents the electrical conductivity of such aqueous silica suspension expressed in microsiemens.cm -1 . In a fourth embodiment, novel silica particulates are compatible with such metal cations as zinc, tin, strontium, and the like, as well as with the fluorides, and have a unique surface chemistry such that the number of OH -  functions thereof, expressed in OH -  /nm 2 , is equal to or less than 10, and also have a zero charge point (ZCP) ranging from 3 to 6.5 and a pH, in aqueous suspension, which varies as a function of the electrical conductivity thereof according to the equation pH=b-a log (D) in which a is a constant equal to or less than 0.6; b is a constant equal to or less than 8.5; and (D) represents the electrical conductivity of such aqueous silica suspension, expressed in microsiemens.cm -1 .

This application is a divisional, of application Ser. No. 08/141,337,filed Oct. 26, 1993, now allowed a divisional of application Ser. No.07/901,078, filed Jun. 19, 1992 now U.S. Pat. No. 5,286,478, which is aContinuation-in-Part of application Ser. Nos. 07/261,935 and 07/261,936,both filed Oct. 25, 1988 both abandoned, and application Ser. Nos.07/518,764 and 07/518,765, both filed May 3, 1990 both abandoned.

CROSS-REFERENCE TO COMPANION APPLICATION

My copending application Ser. No. 07/353,528, filed May 18, 1989,assigned to the assignee hereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to novel silica particulates especiallywell adapted for incorporation into dentifrice compositions, to aprocess for the production of such novel particulates, and to improveddentifrice compositions comprising same.

2. Description of the Prior Art

It is known to this an that silica is a useful material forincorporation into dentifrice compositions. It performs a variety ofdifferent functions therein.

Firstly, it serves as an abrasive agent, thus mechanically contributingto the elimination of dental plaque.

It may also serve as a thickening agent to impart particular rheologicalproperties to the dentifrice, as well as a colorant to impart particularcoloration to the composition.

It is also known to this an that dentifrices contain various activeagents, in particular for the prevention of dental caries, to reduce theformation of dental plaque or the deposition of tartar on the teeth.Among such agents, the fluorides and the zinc compounds are especiallyrepresentative. Other elements are also incorporated, such asphosphates, pyrophosphates, polyphosphates, polyphosphonates,guanidines, in particular the bisbiguanides, and one of the compoundsmost typically included is chlorhexidine. Dentifrice formulations mayalso contain flavorants, perfumes, and the like.

The dentifrice compositions can also contain organic amino compounds. Bythe term "organic amino compounds" is intended any active moleculepresent in the dentifrice formulation and containing at least onenitrogen atom. Particularly representative of such organic aminocompounds are (1) the fluorine-containing amines useful for cariesprophylaxis and especially long-chain amino acid or amino additionproducts with hydrogen fluoride, such as cetyl amine hydrofluoride,bis-(hydroxyethyl)-aminopropyl-N-hydroxyethyl octadecyl aminedihydrofluoride, octadecyl amine fluoride andN,N',N'-tri-(polyoxyethylene)-N-hexadecyl propylene diamenedihydrofluoride; (2) amino oxides useful as nonionic surfactantsprepared by oxidation of tertiary aliphatic amines with hydrogenperoxide, especially the alkyl amine oxides of the formula R(CH₃)₂ N→O,in which R is a straight or branched chain alkyl radical havingapproximately 10 to 24 carbon atoms, and the amine oxides of the formulaR(CH₂ CH₂ OH)₂ N→O, in which R is as defined above; (3) alkyl amines,which can be primary, secondary, tertiary or quaternary aliphatic aminesuseful as cationic surfactants, such as those of the formula R-CH₂ NH₂,or dimethyl alkyl amines of the formula R-N(CH₃)₂ and cetyl trimethylammonium bromide; and (4) alkyl betaines, which are N-alkyl derivativesof N-dimethyl glycine and alkyl amidoalkyl betaines, designatedhereinafter as "alkyl betaines".

Exemplary of this class of amphoteric surfactants are the alkyl betainesof the formula: ##STR1## and the alkyl amidopropyl dimethyl betaines ofthe formula: ##STR2## wherein R is a straight or branched chain alkylradical having 10 to 24 carbon atoms.

A certain number of metal cations can be present in dentifricecompositions. Exemplary thereof are the alkaline earth metal cations,particularly calcium, strontium, barium, cations of Group IIIa,aluminum, indium, cations of Group IVa, germanium, tin, lead and cationsof Group VIII, manganese, iron, nickel, zinc, titanium, zirconium,palladium, and the like. Such cations can be in the form of inorganicsalts, e.g., the chloride, fluoride, nitrate, phosphate or sulfate, orin the form of organic salts, such as the acetate, citrate, and thelike.

More specific examples of such salts are zinc citrate, zinc sulfate,strontium chloride, tin fluoride in the form of the single salt (SnF₂)or the double salt (SnF₂ /KF), stannous chlorofluoride SnClF and zincfluoride (ZnF₂).

The presence of the above-mentioned agents in the dentifrice presentsthe problem of their compatibility with silica. In effect, dueparticularly to its absorbent properties, the latter may have a tendencyto react-with these agents such that they are no longer available toelicit their aforesaid therapeutic and/or useful responses.

French Patent Application 87/15,276 describes silica particulatescompatible with zinc. However, the silicas described do not exhibit acompletely adequate compatibility with other metal cations, such as tin,strontium, and the like.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofa novel silica material that is improvedly compatible with the typicaldentifrice additives: the zinc compounds; the guanidines, particularlythe bis-biguanidines, the most representative of which is chlorhexidine;and the aforementioned organic amino compounds, particularly the classof fluorine-containing amines and betanes.

Another object of the present invention is the provision of a novelsilica material having improved compatibility with the various cationstypically present in dentifrice formulations, such as zinc, strontium,tin, etc.

Another object of the present invention is the provision of processesfor the preparation of such improvedly compatible silica particulates.

Another object of the present invention is the provision of a novelsilica material that is also compatible with the fluoride anion. Asenhanced compatibility with metal cations characteristically reducescompatibility with the fluoride anion, it is therefore particularlyadvantageous that the novel silica material of this invention remainscompatible with the fluoride anion which is present in virtually alldentifrice compositions.

It has now unexpectedly been determined that the desired compatibilityessentially depends on the surface chemistry of the silica particles.Indeed, it has now been established that there must exist a certainnumber of conditions with respect to the surface of the silica particlesto ensure that they are compatible. According to a first embodiment ofthe present invention the silica particulates are characterized in thatthey display compatibility with zinc compounds of at least 50%, and havea number of OH functions, expressed as OH/nm², of at most 15 and a zerocharge point (PZC) ranging from 3 to 6.5.

In a process embodiment specific to this first embodiment, the novelsilica particulates are prepared by reacting a silicate with an acid,whereby a suspension or gel of the silica is produced, then separatingand drying the silica, and thereafter washing the separated silica cakewith water, followed by a second washing or a treatment with an acidsolution.

The silica particulates according to a second embodiment of the presentinvention are characterized in that they display compatibility withcompounds of the guanidine type, and in particular chlorhexidine, of atleast 65% and more preferably at least 90%.

Furthermore, the silica particulates of this second embodiment, whichare compatible with compounds of the guanidine type and in particularwith chlorhexidine, are also characterized in that they have a surfacechemistry such that the acidity function Ho thereof is at least 3.3.

In a process embodiment, the novel silica particulates are prepared byreacting a silicate with an acid, whereby a suspension or a gel ofsilica is produced, then separating and drying the silica, andthereafter washing the separated silica cake with water, until theconductivity of the filtrate is at the most 200 microsiemens.cm⁻¹.

A third embodiment of the invention features novel silica particulates,improvedly compatible with the class of fluorine-containing amines andbetaines. These particulates are distinguished as having a surfacechemistry such that, in aqueous suspension, the pH thereof varies as afunction of its concentration in the area defined by the twoinequations:

    pH≦7.5-0.7 log (C)                                  (Ia)

and

    pH≧5.0-0.5 log (C)                                  (Ib)

and the pH thereof also varying as a function of its electricalconductivity in the area defined by the two inequations:

    pH≦8.5-0.4 log (D)                                  (IIa)

and

    pH≧7.0-0.6 log (D)                                  (IIb)

wherein inequations (Ia) and (Ib), (C) represents the weightconcentration of the aqueous silica suspension, expressed in % SiO₂ ;and wherein inequations (IIa) and (IIb), (D) represents the electricalconductivity of the aqueous silica suspension, expressed inmicrosiemens.cm⁻¹.

The novel silica particulates of this third embodiment of the inventionalso have an acidity function Ho of at least 4.0, a number of OH⁻ sitesper nm² equal to or below 12, and a zero charge point (ZCP) of at least4.

This third embodiment also features novel silica particulates displayinga compatibility of at least 30% with organic amino compounds, moreparticularly at least 50% compatibility and preferably at least 80%compatibility with such organic amino compounds as fluorine-containingamines, amine oxides, alkyl amines and alkyl betaines.

This third embodiment also features novel silica particulates displayinga compatibility of at least 50%, more particularly at least 70%, withmetal cations.

This third embodiment also features novel silica particulates displayinga compatibility with compounds of guanidine type, in particularchlorhexidine, of at least 30% and more particularly at least 60%.

The third embodiment also features a process for the preparation of suchnovel silica particulates, comprising reacting a silicate with an acidto produce a silica suspension or a silica gel, next conducting a firstaging step at a pH equal to or above 6 and equal to or below 8.5,followed by a second aging at a pH equal to or below 6 and then a thirdaging step at a pH equal to or below 5, next separating the silica andwashing it with water to such extent that an aqueous suspension isproduced having a pH, measured on a 20% SiO₂ suspension, in accordancewith the following equation:

    pH=d-e log (D)                                             (III)

in which e is a constant equal to or greater than 0.6 and equal to orless than 1.0; d is a constant equal to or greater than 7.0 and equal toor less than 8.5; and (D) represents the electrical conductivity of theaqueous silica suspension, expressed in microsiemens.cm¹, and lastlydrying such aqueous suspension.

A fourth embodiment of the present invention features silicaparticulates improvedly compatible with zinc and other metal cations,which are characterized in that they have a surface chemistry such thatthe number of OH⁻ functions thereof, expressed in OH⁻ /nm², is equal toor less than 10, that its zero charge point (ZCP) ranges from 3 to 6.5and that, in aqueous suspension, the pH thereof varies as a function ofits electrical conductivity according to the following equation (I):

    pH=b-a log (D)                                             (I)

wherein a is a constant equal to or less than 0.6; b is a constant equalto or less than 8.5; and (D) represents the electrical conductivity ofthe aqueous silica suspension, expressed in microsiemens.cm⁻¹.

The novel silica particulates of this fourth embodiment display at least30% compatibility with at least one divalent and higher valency metalcation selected from Groups IIa, IIIa, IVa and VIII of the PeriodicTable, more particularly at least 50% compatibility and preferably atleast 80% compatibility.

The fourth embodiment also features a process for the preparation ofsuch novel silica particulates, comprising reacting a silicate with anacid, thus providing a silica gel or suspension, next conducting a firstaging step at a pH equal to or above 6 and equal to or less than 8.5,followed by a second aging at a pH equal to or less than 5.0, thenseparating the silica and washing it with hot water to such extent thatan aqueous suspension is produced having a pH, measured on a 20% SiO₂suspension, in accordance with the following equation:

    pH=d-c log (D)                                             (II)

wherein c is a constant equal to or less than 1.0; d is a constant equalto or less than 8.5; and (D) represents the electrical conductivity ofthe aqueous silica suspension, expressed in microsiemens.cm⁻¹ and lastlydrying such aqueous suspension.

The present invention also features improved dentifrice compositionscomprising the novel silica particulates described in the aboveembodiments, or prepared by any of the aforesaid process embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The invention includes several embodiments of novel silica particulatesand processes for the production thereof.

Embodiment I

A first embodiment of the present invention features novel silicaparticulates especially compatible with zinc compounds typically used asdentifrice additives.

More particularly, as indicated above, the essential characterizingfeatures of the subject novel silica particulates reside in the surfacechemistry thereof. Thus, surface acidity is an important aspect.Relative to such acidity, one of the distinguishing characteristics ofthe silica particulates of the invention is the number of their surfaceacid sites.

This number may be measured as the number of OH or silanol groups pernm².

Such number is determined as follows:

The number of OH sites on the surface is associated with the amount ofwater released by silica at temperatures 190° C. and 9000° C.

The silica specimens are initially dried at 105° C. for 2 hours.

A mass Po of silica is placed in a thermobalance and heated to 190° C.for 2 hours; the mass obtained is P₁₉₀. The silica is then heated to900° C. for 2 hours; the new mass obtained is P₉₀₀.

The number of OH sites is calculated by the following equation: ##EQU1##wherein NOH is the number of OH sites per nm² of the surface, and A isthe (BET) specific surface of the solid, in m² /g.

In the present embodiment, the silica particulates of the inventionadvantageously have a number of OH/nm² less than or equal to 15, moreparticularly a maximum of 12, and preferably ranging from 3 to 12.

The nature of the OH sites of the silica particulates of the invention,which is also a characteristic of their surface chemistry (pH of thesurface), too may be determined by the point of zero charge.

The point of zero charge (PZC) is defined by the pH of a suspension ofsilica for which the electrical charge of the surface of the solids iszero, regardless of the ionic strength of the medium. This PZC measuresthe real pH of the face, to the extent that it is free of impurities ofthe ionic type.

The electrical charge is determined by potentiometry. The principle ofthe method is based on the total balance of protons adsorbed or desorbedon the surface of the silica at a given pH.

By means of the equations describing the total balance of the operation,it is easy to show that the electrical charge C of the surface,considered relative to a corresponding reference, has a zero surfacecharge given by the equation: ##EQU2## wherein: A is the specificsurface of the solids, in m² /g;

M is the mount of solids in the suspension, in g;

F is the Faraday constant;

H or OH represents the variation per unit of the surface of the excessof H⁺ or OH⁻ ions, respectively, of the solids.

The experimental procedure of the determination of PZC is the following:

The method described by Berube and Bruyn, J. Colloid Interface Sc., 27,305 (1968) is used.

The silica is initially washed in high resistivity deionized water (10Mega.Ohm.cm), dried and degassed.

In actual practice, a series of solutions at pHo 8.5 is prepared by theaddition of KOH or HNO₃ and containing an electrolyte (KNO₃) in aconcentration of from 10⁻⁵ to 10⁻¹ mole/l.

To these solutions, a given mass of silica is added and the pH of theresulting suspensions is permitted to stabilize under agitation, at 25°C. and under nitrogen, for 24 h; its value is the pH'o.

The standard solutions are the supernatants obtained by centrifugationfor 30 min at 1,000 rpm of a fraction of the same suspensions; the pH'ois the pH of these supernatants.

The pH of a known volume of these suspensions and of correspondingstandard solutions is adjusted to pHo by adding the necessary amount ofKOH and the suspensions and standard solutions are permitted tostabilize for 4 hours.

V_(OH). N_(OH) is the number of equivalents of base added to change fromthe pH'o to pHo of a known volume (V) of the suspension of the standardsolution.

The potentiometric analysis of the suspensions and the standardsolutions is carried out from the pHo by the addition of nitric acid toa pHf=2.0.

Preferably, acid is added incrementally corresponding to a variation ofthe pH by 0.2 pH units. After each addition, the pH is stabilized toattain pHf.

Beginning with pHo, the (V_(H). N_(H) - V_(OH). N_(OH)) is plotted as afunction of the pH increments for all of the suspensions (at least 3ionic strengths) and for all of the corresponding standard solutions.

For each value of pH (no 0.2 unit), the difference between theconsumption of H⁺ or OH⁻ for the suspension and the correspondingstandard solution is then established. This operation is repeated forall ionic strengths.

This gives the (H - OH) corresponding to the consumption of the protonsof the surface. The surface charge is calculated by the above equation.

Subsequently, the curves of the surface charge are calculated as afunction of the pH for all of the ionic strengths considered. The PZC isdefined by the intersection of the curves.

The silica concentration is then adjusted as a function of its specificsurface.

For example, 2% suspensions are used for 50 m² /g silica at 3 ionicstrengths (0.1, 0.01 and 0.001 mole/l).

100 ml of the suspension are analyzed by using 0.1M potassium hydroxide.

The PZC of the silica particulates of the present invention ranges from3 to 6.5.

To further improve compatibility, in particular relative to elementsother than zinc, in particular fluorine, it is advantageous to limit thealuminum content of the silica of the invention to a maximum of 500 ppm.

The maximum iron content of the silica of the invention should be 200ppm.

The maximum calcium content may be 500 ppm and more particularly 300ppm.

The silica of the invention preferably has a maximum carbon content of50 ppm and more particularly of 10 ppm.

The pH of the silica according to the invention measured by the NFT(French National Standard) standard 45-007 is generally at most 7. Moreparticularly, it ranges from 5.5 to 7, and preferably from 6.0 to 7.0.

These characteristics make it possible to obtain silica particulatesthat are compatible with zinc compounds. This compatibility, measured bythe test described below, is at least 50%, preferably at least 80%, andmore preferably at least 90%. Depending upon the particular case, thesilica particulates of the invention are also compatible with fluorides,phosphates and derivatives thereof.

In addition to the chemical surface properties described above, whichimpart compatibility thereto, the silica particulates of the inventionhave physical properties which are perfectly suited for their use indentifrices. These structural characteristics are described as follows.

Advantageously, the BET surface of the silica particulates of theinvention ranges from 40 to 600 m² /g, and more preferably from 40 to350 m² /g. Their CTAB surface typically ranges from 4 to 400 m² /g, andmore preferably from 40 to 200 m² /g.

The BET surface is determined by the BRUNAUER-EMMET-TELLER methoddescribed in the Journal of the American Chemical Society, Vol. 60, p.309 (February 1938) and according to the standard NF X11-622 (3.3).

The CTAB surface is the external surface determined by the ASTM standardD3785, but by using the adsorption of hexadecyltrimethyl ammoniumbromide (CTAB) at pH 9 and taking 35 A⁰² as the projected area of theCTAB molecule.

The silica of the invention may correspond to the three types usuallydistinguished in the dentifrice field.

Thus, the silica particles of the invention may be of the abrasive type.Same then have a BET surface of from 40 to 300 m² /g. In this case, theCTAB surface ranges from 40 to 100 m² /g.

The silica particles of the invention may also be of the thickeningtype. Their BET surface then ranges from 120 to 450 m² /g, and morepreferably from 120 to 200 m² /g. They may have a CTAB surface of from120 to 400 m² /g, and more preferably from 120 to 200 m² /g.

Finally, as a third type, the silica particles of the invention may bebifunctional. In this instance they have a BET surface of from 80 to 200m² /g. Their CTAB surface ranges from 80 to 200 m² /g.

The silica particles of the invention may also exhibit an oil uptake offrom 80 to 500 cm³ /100 g determined by the NFT standard 30-022 (March53) using dibutyl phthalate.

More precisely, such oil uptake ranges from 100 to 140 cm³ /100 g forthe abrasive silica, from 200 to 400 for the thickening silica and from100 to 300 for the bifunctionals.

The silica particulates preferably have, again vis-a-vis theirdentifrice applications, a particle size of from 1 to 10 μm.

This mean particle size is measured by Counter-Coulter.

The apparent density thereof generally ranges from 0.01 to 0.3. In apreferred embodiment of the invention, the silica particulates areprecipitated silica particulates.

Finally, the silica of the invention has a refraction index generallyfrom 1.440 to 1.465.

Process for Preparation of Novel Silica Particulates

The process for the preparation of the silica of the invention will nowbe described in greater detail.

As indicated above, the process is of the type comprising reacting asilicate with an acid, resulting in the formation of a suspension or gelof silica.

It will be appreciated that any known operation may be used to preparethis suspension or gel (addition of acid to the base of a vat of silica,simultaneous total or partial addition of the acid and the silicate tothe base of a water vat, or a solution of silicate, etc.), with theselection being made essentially as a function of the physicalcharacteristics of the silica to be produced. It may be advantageous toadjust the pH of the resulting suspension or gel to a value of at most 6and preferably ranging from 4 to 6.

The silica is then separated from the reaction medium by any knownmeans, for example vacuum filtration or filter press.

A silica filter cake is recovered.

In a primary characteristic of the present embodiment, this filter cakeis subjected to a first washing with water, advantageously withdeionized water.

The silica particulates are next subjected to a second washing withwater, or are treated with an acid solution.

The purpose of the second wash, or acid treatment, is to provide silicaparticulates having a pH of at most 7, preferably a pH ranging from 5.5to 7, and more preferably a pH ranging from 6.0 to 7.0, as well as a PZCranging from 3 to 6.5.

The acid solution may be, for example, a solution of an inorganic acid,such as nitric acid.

However, in a preferred embodiment of the invention, the acid solutionmay also be a solution of an organic acid, in particular a complexingorganic acid. Such an acid is advantageously selected from amongcarboxylic, dicarboxylic, hydroxycarboxylic and aminocarboxylic acids.

Exemplary of such acids is acetic acid, and exemplary of the complexingacids are tartaric, maleic, glyceric, gluconic and citric acids.

The second wash, or the treatment with acid, may be carried out bypouring the acid solution over the filter cake, or introducing it intothe suspension obtained by the comminution or grinding of the cake. Suchwash or acid treatment is conducted under conditions as to providesilica particulates having the aforesaid final pH value; the pH of thesuspension or medium, prior to drying, must range from 4 to 6, andpreferably from 5 to 6.

It may be advantageous, especially in the case in which a solution of amineral acid is used, to conduct a final wash with deionized water.

In another embodiment of the invention, following the acid/silicatereaction and immediately before the separation of the silica, thesuspension or gel is aged. This aging is typically carried out at amaximum pH of 6, for example, at a value of from 4 to 6.

It is also possible to carry out the aging during the reaction, forexample at a pH of from 6 to 8. The aging is preferably conducted at anelevated temperature, for example a temperature of from 80° to 100° C.,and for a period of time ranging from fifteen minutes to two hours.

After the cake is washed and treated as described above, the cake, or ifit is comminuted, the suspension is dried by any known means. Inparticular, drying is by atomization. The dried product is ground, ifnecessary, to obtain the grain size distribution desired.

Improved Dentifrice Compositions

This invention also features improved dentifrice compositions containingthe above novel silica particulates, advantageously prepared by theaforesaid distinct processes.

The amount of silica incorporated into such improved dentifricecompositions may vary over wide limits, but typically it ranges from 5to 35% by weight.

The silica particulates of the invention are well adapted forincorporation into dentifrice compositions comprising at least oneelement selected from among the fluorides, phosphates, and zinc.

As regards the fluoride compounds, the amount thereof preferablycorresponds to a fluorine concentration in the ultimate composition offrom 0.01 to 1% by weight, notably from 0.1 to 0.5% by weight. Thepreferred fluoride compounds are the salts of monofluorophosphoric acid,and in particular those of sodium, potassium, lithium, calcium, aluminumand ammonium, mono- and difluorophosphate, as well as the variousfluorides containing fluorine in the form of a bonded ion, particularlyalkaline fluorides, such as those of sodium, potassium, lithium,ammonium fluoride, stannous fluoride, manganese fluoride, zirconiumfluoride, aluminum fluoride, together with addition products of thesefluorides with each other or with other fluorides, such as potassium,sodium or manganese fluorides.

Other fluorides may also be incorporated in the dentifrices of thepresent invention, such as, for example, zinc fluoride, germaniumfluoride, palladium and titanium fluorides, and alkaline fluozirconates,such as, for example, of sodium or potassium, stannous fluozirconate,and sodium or potassium fluoborate or fluosulfates.

Organic fluorine compounds may also be incorporated, preferably knowncompounds such as the addition products of amines and long chainaminoacids with hydrogen fluoride, cetylamine fluoride, thedihydrofluoride or bis(hydroxyethyl)aminopropyl-N-hydroxyethyloctadecylamine, octadecylamine fluoride and the dihydrofluoride ofN,N',N'tri-(polyoxyethylene)-N-hexadecylpropylenediamine.

Zinc is incorporated, in particular, in the form of its citrate orsulfate.

As elements that are useful as anti-plaque agents of the polyphosphateor polyphosphonate, guanidine, or bisbiguanide type, those set forth inU.S. Pat. Nos. 3,934,002 and 4,110,083 are representative.

The subject dentifrice compositions may also comprise a binder.

The principal binders are selected from among:

(i) Cellulose derivatives: methylcellulose, hydroxyethyl cellulose,sodium carboxymethylcellulose;

(ii) Mucilages: carraghenates, alginates, agar-agar and geloses;

(iii) Gums: arabic and tragacanth gums, xanthan gum, Karaya gum;

(iv) Carboxyvinyl and acrylic polymers;

(v) Polyoxyethylene resins.

In addition to the silica particulates of the invention, the dentifricecompositions may contain one or more other abrasive polishing agentsselected from among:

(i) Precipitated calcium carbonate;

(ii) Magnesium carbonate;

(iii) Di- and tricalcium phosphates;

(iv) Insoluble sodium metaphosphate;

(v) Calcium pyrophosphate;

(vi) Titanium dioxide (whitening agent);

(vii) Silicates;

(viii) Alumina and silicoaluminates;

(ix) Zinc and tin oxides;

(x) Talc;

(xi) Kaolin.

These dentifrice compositions may also contain detergents, humectants,aromatics, sweeteners and colorants and preservatives.

The principal detergents are selected from among:

(i) Sodium laurylsulfate;

Sodium laurylether sulfate and laurylsulfoacetate;

(iii) Sodium dioctylsulfosuccinate;

(iv) Sodium laurylsarcosinate;

(v) Sodium ricinoleate;

(vi) Monoglycerine sulfates.

The principal humectants are selected from among the polyalcohols, suchas:

(i) Glycerol;

(ii) Sorbitol, generally in a 70% solution in water;

(iii) Propylene glycol.

The principal aromatics are selected from among: essences of anise,chinese anise, mint, juniper berry, cinnamon, cloves and roses.

The principal sweetening agents are orthosulfobenzoic imides andcyclamates.

The principal colorants are those selected from among:

(i) Red and rose colorants: amaranth, azorubin, catechou, new coccine(PONCEAU 4 R), cochineal, erythrosine;

(ii) Green colorants: chlorophyll and chlorphylline;

(iii) Yellow colorants: sun yellow (Orange S) and quinoline-yellow.

The principal preservatives are: parahydroxy benzoates, formaldehyde andproducts releasing same, hexetidine, quaternary ammonium compounds,hexachlorophene, bromophene and hexamedine.

Finally, the dentifrice compositions may contain therapeutic agents,principally selected from among:

(i) Antiseptics and antibiotics;

(ii) Enzymes;

(iii) oligoelements and the fluorine compounds described above.

Illustrative Examples

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

In said examples to follow, the tests described immediately below werecarried out to measure the compatibility of the silica with variouscompounds. ##EQU3## Measurement of compatibility with fluorides;

4 g silica were dispersed in 16 g of a 0.3% solution of sodium fluoride(NaF). The suspension was agitated for 24 h at 37° C. Aftercentrifugation at 20,000 rpm for 30 min, the supernatant was filtered ona 0.2 μm Millipore filter. The solution obtained in this mannerconstituted the test solution.

A reference solution was prepared by the same procedure, but without thesilica.

The compatibility with fluorides was determined by the % of freefluoride measured by a fluoride selective electrode (Orion).

It was determined by the following relationship: ##EQU4## Measurement ofcompatibility with zinc:

4 g silica were dispersed in 100 ml of a 0.06% aqueous solution ofZnSO₄.7 H₂ O. A suspension was obtained, the pH of which was stabilizedat 7 in 15 min by the addition of NaOH or H₂ SO₄. The suspension wasthen agitated for 24 h at 37° C. and centrifuged at 20,000 rpm for 30min.

The supernatant, filtered on a 0.2 μm Millipore filter, constituted thetest solution.

A reference solution was prepared by the same procedure, but without thesilica.

The concentration of free zinc in the two solutions was determined byatomic absorption (214 nm).

The compatibility was determined by the following relationship: ##EQU5##Measurement of compatibility with sodium and potassium pyrophosphates:

4 g silica were dispersed in 16 g of a 1.5% aqueous suspension of sodiumor potassium pyrophosphate. The suspension was agitated for 24 h at 37°C., then centrifuged at 20,000 rpm for 30 min.

The supernatant was filtered on a 0.2 μm Millipore filter. 0.2 g of thesolution, diluted in 100 ml water in a volumetric flask, constituted thetest solution.

A reference solution was prepared by the same procedure, but without thesilica.

The free pyrophosphate ion (P₂ O₇ ⁼) concentration of the two solutionswas determined by ionic chromatography (DIONEX 2000i system), equippedwith an integrator.

The compatibility was determined from the areas of the peaks obtained inthe chromatograms and corresponding to the retention time of thepyrophosphate in the test and reference solutions. ##EQU6##

EXAMPLE 1

Into a reactor equipped with a temperature and pH control system and aturbine agitation system, 6 l of deionized water were introduced.

After commencing agitation (300 rpm), the contents of the reactor wereheated to 85° C.

When this temperature was reached, the following materials weresimultaneously added: 8.5 l sodium silicate having a silicaconcentration of 120 g/l and a SiO₂ /Na₂ O ratio of 3.5, at a flow rateof 0.34 l/min, and 13.5 l sulfuric acid having a concentration of 80g/l. The acid flow rate was adjusted such that the pH of the medium wasmaintained at a constant value 8.0.

After 40 min of addition, the mixture was aged for 10 min at this pH andtemperature.

The addition of the silicate was discontinued and the addition of theacid continued until the pH of the reaction mixture was stabilized at 4.

The mixture was then aged for 15 min at this pH and at 85° C.

It was subsequently filtered and the moist filter cake was washed withdeionized water.

The filter cake was then dispersed in deionized water to form ahomogeneous suspension having a silica concentration of 50 g/l. The pHof this suspension was adjusted to 5.8 by the addition of nitric acidand it was permitted to stabilize at this pH for 15 min.

The suspension was filtered.

The product was then dried by atomization and ground in a forplex typegrinder to produce a grain size of 9 microns.

The physico/chemical properties of the resulting silica particulateswere as follows:

    ______________________________________                                        (i)     BET surface      90     m.sup.2 /g                                    (ii)    CTAB surface     60     m.sup.2 /g                                    (iii)   Oil uptake       105    cm.sup.3 /100 g                               (iv)    pH               6.8                                                  (v)     Number of OH/nm.sup.2                                                                          8.                                                   ______________________________________                                    

The chemical analyses of the silica are reported in the following table:

    ______________________________________                                        Ions      Al     Fe           Ca   C                                          ______________________________________                                        ppm       350    110          300  10                                         ______________________________________                                    

The PZC of the silica was 4.5.

In the following table the different compatibilities of the silicaparticles with the various ingredients of a dentifrice formulation andmeasured by the different tests described above, are set forth:

    ______________________________________                                                    Fluoride  Pyrophosphate                                                                              Zinc                                       Ingredients (NaF)     (Na/K)       (ZnSO.sub.4)                               ______________________________________                                        % Compatibility                                                                           90        98           90                                         ______________________________________                                    

COMPARATIVE EXAMPLE 2

As a comparison, compatibility measurements were made using thecommercial silica typically employed in dentifrice formulations, andthese are reported below, together with the physico/chemical propertiesthereof:

    ______________________________________                                        Silica                                                                        (trademark of                                                                          Surfaces                                                             manu-    (m.sup.2 /g)                                                                             Number   Compatibilities                                  facturer)                                                                              CTAB    BET    OH/nm.sup.2                                                                          Zn  F   Pyrophosphate                          ______________________________________                                        Zeodent  50      100    30     0   95  95                                     (Hubert)                                                                      Tixosil 53                                                                             50      250    30     0   60  90                                     (Rhone-                                                                       Poulenc)                                                                      Z 119    50      60     25     20  95  95                                     (Rhone-                                                                       Poulenc)                                                                      ______________________________________                                    

It should be noted that for the silica of this table the PZC was lessthan 3.

EXAMPLE 3

This example relates to the formulation of an opaque dentifrice of thepaste type:

Its formula was the following:

    ______________________________________                                        Glycerin                22.00                                                 CMC 7mFD                1.00                                                  Sodium saccharinate     0.20                                                  Sodium monofluorophosphate                                                                            0.76                                                  Sodium fluoride         0.10                                                  Sodium lauryl sulfate (30% aqueous)                                                                   4.67                                                  Sodium benzoate         0.10                                                  Perfume                 0.90                                                  Titanium dioxide        1.00                                                  Silica of Example 1     31.50                                                 ZnSO.sub.4.7H.sub.2 O   0.48                                                  Distilled water         37.29                                                 ______________________________________                                    

Rheological testing and visual examination of the above paste dentifriceevidenced that the conventional properties thereof were good.

Embodiment II

According to a second embodiment of the present invention, the silicaparticulates have improved compatibility with guanidine and surfaceacidity is an important aspect. Relative to surface acidity, one of thedistinguishing characteristics of the silica particulates of theinvention is the strength of their surface acid sites.

By the term "acidity" is intended acidity in the sense of a Lewis acid,i.e., connoting the tendency of one site to accept a pair of electronsfrom a base according to the equilibrium:

    B:+A=BA

In characterizing the silica particulates of the invention, theconvention of "acidity function", Ho, developed by Hammett, is used tomeasure the tendency of the acid, silica in this case, to accept a pairof electrons from base.

The Ho function is defined by the conventional relationship: ##EQU7##

To determine the strength of the acid sites of a silica of the inventionby the Hammett method, the indicator technique is used, describedoriginally by Wailing, J. Am. Chem. Soc., 72, 1164 (1950).

The strength of the acid sites is determined by colored indicators, andthe pK_(a) of transfer between the acid and basic states under theconditions of use, which are known.

Thus, the lower the pK_(a) of the indicator undergoing the change incolor, the stronger the acidity of the site. The following tablecompiles, for exemplary purposes, a nonlimiting list of Hammettindicators suitable for use in circumscribing the value of Ho, bydetermining in which form two successive indicators are adsorbed.

                  TABLE I                                                         ______________________________________                                                          Color                                                                           Basic    Acid                                             Indicator           Form     Form    pK.sub.a                                 ______________________________________                                        Neutral red         yellow   red     +6.8                                     Methyl red          yellow   red     +4.8                                     Phenylazonaphthylamine                                                                            yellow   red     +4.0                                     p-Dimethylaminoazobenzene                                                                         yellow   red     +3.3                                     2-Amino-5-azotoluene                                                                              yellow   red     +2.0                                     Benzene azodiphenylamine                                                                          yellow   red     +1.5                                     4-Dimethylaminozao-1-naphthalene                                                                  yellow   red     +1.2                                     Crystal violet      blue     yellow  +0.8                                     p-Nitrobenzene azo-(p'-nitro)                                                                     orange   violet  +0.43                                    diphenylamine                                                                 Dicinnamalacetone   yellow   red     -3.0                                     Benzalacetophenone  colorless                                                                              yellow  -5.6                                     Anthraquinone       colorless                                                                              yellow  -8.2                                     ______________________________________                                    

The color of the indicators adsorbed onto the silica is a measure of thestrength of the acid sites. If the color is that of the acid form of theindicator, the value of the Ho function of the surface is equal to orless than the pK_(a) of the indicator.

Low values of Ho correspond to high strengths of the acid sites.

Thus, for example, a silica giving a red color withp-dimethylaminoazobenzene and yellow with 2-amino-5-azotoluene wouldhave an acid function Ho ranging from 3.3 to 2.

Experimentally, the determination is carried out with 0.2 g silicaplaced in a test tube in the presence of an indicator solution, in aconcentration of 100 rag/l in cyclohexane.

The silica is initially dried at 190° for 2 hours and maintained, to theexclusion of humidity, in a desiccator.

By means of agitation, the adsorption if it takes place, is produced ina few minutes and the change in color is visible to the naked eye or mayoptionally be observed by studying the characteristic absorption spectraof the color indicators adsorbed, both in their acid and their basicforms.

A first characteristic of the silica particulates of the invention isthat they have an acidity function, determined as described above, of atleast 3.3.

The strength and the nature of the surface acid sites may also bemeasured by the infrared spectrometry of pyridine adsorbed onto thesilica.

It is known that the amount of pyridine adsorbed onto a solid mass makesit possible to determine, in particular, the nature of the surface acidsites.

Pyridine is a relatively strong base (pK_(b) =5).

The formation of the pyridinium ion (PyH+) further permitsdifferentiation of sites of the Lewis and Bronsted type.

Information concerning the acidity of a surface of a solid may also beobtained by studying the absorption bands of pyridine in the range of1,700 cm⁻¹ to 1,400 cm⁻¹.

Furthermore, the value of the deviation of the characteristic bands ofpyridine and its ionized forms, before and after adsorption, makes itpossible to quantify the strength of the acid sites.

The pyridinium ion gives a band at 1,540 cm⁻¹, while pyridine bonded byH bonds or by coordination, gives a band in the range of 1,400-1,465cm⁻¹. It also appears that the pyridine band located at 1,583 cm⁻¹ isdisplaced, if the pyridine is adsorbed. This band indicates the presenceof Lewis acid sites. The acid strength of the latter is proportional tothe displacement of the band.

In summary, it is possible to use the bands of 1,540, 1,650 and 1,485cm⁻¹ to define acidity of the Bronsted type, and the range of1,440-1,465 cm⁻¹ for Lewis acidity.

Experimentally, the measurements are carried out using a silicasuspension in carbon tetrachloride, in the presence of pyridine.

The silica is first dried at 190° C. for 2 hours, and maintained withthe exclusion of humidity. After cooling, 1 g silica is dispersed bymagnetic agitation followed by ultrasonic dispersion (10 min) in 50 mlCCl₄.

0.8 mg pyridine is added per square meter of the silica introduced. Thedispersion is heated at reflux, under agitation, for 1 hour.

The same procedure is used to prepare a control solution of the samepyridine concentration, but without silica, and a control solution ofthe same silica concentration, but without pyridine.

The adsorption spectra of pyridine is determined by infraredspectroscopy of the suspension, the solution of pyridine without silicaand the silica suspension without pyridine.

From the spectra obtained from the suspension, the spectra correspondingto the control solution and the spectra corresponding to the controlsuspension, are subtracted.

The silica is characterized by the position of the remaining bands andthe displacement of the pyridine and pyridium ion absorption bandsrelative to the position of the bands of their unadsorbed forms.

In general, the spectra obtained should not display the pyridinium peak(band at 1,540 cm⁻¹), the absence of the peak indicating that the silicahas an acid function Ho of at least 3.3.

The magnitude of the displacement of the pyridine and pyridine adsorbedmakes it possible to determine the Acidity of the surface acid sites.Typically, for the band of 1,440 cm⁻¹ this displacement (Δν) should beat most 10 cm⁻¹, more particularly 5 cm⁻¹ at most.

In a preferred embodiment of the invention, this Δν is zero.

The silica described above displays good compatibility withchlorhexidine; this compatibility, measured by the test described above,should be at least 65%, in particular at least 80% and preferably atleast 90%.

However, in another preferred embodiment of the invention, the silica isalso compatible with fluorine values. In this case, there is a maximumcontent of anions of the type of SO₄ ²⁻, Cl⁻, NO₃ ⁻, PO₄ ³⁻, CO²⁻, of5×10⁻³ moles per 100 g silica.

This compatibility becomes greater with declining values of suchcontent. Preferably, it will be a maximum of 1×10⁻³ moles and morepreferably 0.2×10⁻³ per 100 g silica.

In the case of silica prepared from sulfuric acid, this anion content ismore conveniently expressed as content in SO₄, by weight. In this case,the maximum content is 0.5%.

In another preferred embodiment of the invention, such maximum contentis 0.1% and more preferably 0.02%.

Such compatibility may be improved further, in particular relative tocertain elements such as zinc, by observing the conditions of the numberof acid sites of the surface. This number may be measured as the numberof OH or silanol groups per nm², and is determined as described above inEmbodiment I.

In the present embodiment, the silica particulates advantageously have anumber of OH/nm² less than or equal to 15, more particularly a maximumof 12, and preferably ranging from 3 to 12.

The nature of the OH sites of the silica particulates, which is alsocharacteristic of their surface chemistry, may also be determined by thepoint of zero charge (PZC). The method for determining the PZC is asdescribed in Embodiment I.

In actual practice, it is preferable that the value of said PZC be atleast 3 and more particularly from 4 to 6. In the case of bettercompatibility with zinc, it is 6.5 at maximum. For fluorinecompatibility, a maximum PZC of 7 is preferred.

To further improve compatibility, in particular relative to fluorine, itis advantageous to limit the aluminum content of the silica of theinvention to a maximum of 500 ppm.

The maximum iron content of the silica of the invention should be 200ppm.

The maximum calcium content may be 500 ppm and more particularly 300ppm.

The silica of the invention preferably has a maximum carbon content of50 ppm and more particularly of 10 ppm.

The pH of the silica according to the invention measured by the NFT(French National Standard) standard 45-007 is generally at most 8. Moreparticularly, it ranges from 6.0 to 7.5.

These characteristics make it possible to obtain silica particulatesthat are compatible with at least the guanidines and in particularchlorhexidine, and, depending upon the particular case, also withfluorides, phosphates and their derivatives and particularly zinc.

Advantageously, the BET surface of the silica particulates of theinvention ranges from 40 to 600 m² /g, and more preferably from 40 to350 m² /g. Their CTAB surface typically ranges from 4 to 400 m² /g, andmore preferably from 40 to 200 m² /g.

In addition to the chemical surface properties described above, whichimpart compatibility thereto, the silica particulates of the inventionhave physical properties which are perfectly suited for their use indentifrices. These structural characteristics are described as follows.

The BET surface is determined by the BRUNAUER-EMMET-TELLER methoddescribed in the Journal of the American Chemical Society, Vol. 60, p.309 (February 1938) and according to the standard NF X11-622 (3.3).

The CTAB surface is the external surface determined by the ASTM standardD3785, but by using the adsorption of hexadecyltrimethyl ammoniumbromide (CTAB) at pH 9 and taking 35 Å² as the projected area of theCTAB molecule.

The silica of the invention may correspond to the three types usuallydistinguished in the dentifrice field.

Thus, the silica particles of the invention may be of the abrasive type.Same then have a BET surface of from 40 to 300 m² /g. In this case, theCTAB surface ranges from 40 to 100 m² /g.

The silica particles of the invention may also be of the thickeningtype. Their BET surface then ranges from 120 to 450 m² /g, and morepreferably from 120 to 200 m² /g. They may have a CTAB surface of from120 to 400 m² /g, and more preferably from 120 to 200 m² /g.

Finally, as a third type, the silica particles of the invention may bebifunctional. In this instance they have a BET surface of from 80 to 200m² /g. Their CTAB surface ranges from 80 to 200 m² /g.

The silica particles of the invention may also exhibit an oil uptake offrom 80 to 500 cm³ /100 g determined by the NFT standard 30-022 (March53) using dibutyl phthalate.

More precisely, such oil uptake ranges from 100 to 140 cm³ /100 g forthe abrasive silica, from 200 to 400 for the thickening silica and from100 to 300 for the bifunctionals.

The silica particulates preferably have, again vis-a-vis theirdentifrice applications, a particle size of from 1 to 10 μm.

This mean particle size is measured by Counter-Coulter.

The apparent density thereof generally ranges from 0.01 to 0.3. In apreferred embodiment of the invention, the silica particulates areprecipitated silica particulates.

Finally, the silica of the invention has a refraction index generallyfrom 1.440 to 1.465.

Process for Preparation of Novel Silica Particulates

The process for the preparation of the silica of this embodiment of theinvention will now be described in greater detail.

As indicated above, the process is of the type comprising reacting asilicate with an acid, resulting in the formation of a suspension or gelof silica.

It will be appreciated that any known operation may be used to preparethis suspension or gel (addition of acid to the base of a vat of silica,simultaneous total or partial addition of the acid and the silicate tothe base of a water vat, or a solution of silicate, etc.), with theselection being made essentially as a function of the physicalcharacteristics of the silica to be produced. It may be advantageous toadjust the pH of the resulting suspension or gel to a value of at most 6and preferably ranging from 4 to 6.

The silica is then separated from the reaction medium by any knownmeans, for example vacuum filtration or filter press.

A silica filter cake is recovered.

The process of the invention may then be carried according to twoprincipal variants.

The first variant features the preparation of silica particulatesessentially completely compatible with the guanidines and specificallywith chlorhexidine.

In this case, the process includes a washing of the filter cake. Washingis with water, generally deionized water, until a wash filtrate having aconductivity of less than 200 microsiemens.cm⁻¹ (μS/cm) is obtained.

If it is desired to further improve the compatibilities of the silicaobtained by such process, the washing is continued to a greater extent.

In particular, in a preferred embodiment of the invention, the washingis continued until a conductivity of the water of maximum 100microsiemens.cm⁻¹ is obtained.

After the cake is washed as described above, the cake, or if it iscomminuted, the suspension, is dried by any known means. In particular,drying is by atomization. The dried product is ground, if necessary, toobtain the grain size distribution desired.

The second variant of the process features the preparation of silicaparticulates compatible with other elements, such as fluorine, zinc andphosphates, in addition to the guanidines.

In this variant, the process again includes washing with deionizedwater, as in the first variant. However, this washing may be lessextensive. It may be continued, for example, until a filtrate having aconductivity of maximum 2,000 microsiemens.cm⁻¹ is obtained.

In this second variant, following the first washing operation, a secondwashing, or a treatment of the filter cake with an acid solution oracidulated water, is carried out. This second washing or treatment iscarried out such that a silica is produced having a maximum pH of 8,preferably ranging from 6.0 to 7.5, and a PZC of at least 3, andpreferably from 4 to 6.

This washing or treatment may be carried out by pouring the acidsolution over the filter cake, or introducing it into the suspensionobtained by the comminution of the cake.

The acid washing or treatment is carried out under conditions such that,in order to produce a silica having the pH values indicated above, thepH of the suspension or medium prior to drying must range from 4 to 8,in particular from 5 to 8 and preferably from 6 to 7.

This acid solution may be, for example, a solution of an inorganic acid,such as nitric acid.

However, in another preferred embodiment of the invention, the acidsolution may also be a solution of an organic acid, in particular acomplexing organic acid. Such acid is advantageously selected from amongcarboxylic, dicarboxylic, hydroxycarboxylic and aminocarboxylic acids.

Exemplary of such acids is acetic acid, and exemplary of the complexingacids are tartaric, maleic, glyceric, gluconic and citric acids.

It may be advantageous, especially in the case in which a solution of amineral acid is used, to conduct a final wash with deionized water.

Following the washes or treatments according to the second variant,drying is carried out in the manner described for the first variant.

In another embodiment of the invention, following the acid/silicatereaction and immediately before the separation of the silica, thesuspension or gel is aged. This aging is typically carried out at amaximum pH of 6, for example, at a value of from 4 to 6.

It is also possible to carry out the aging during the reaction, forexample at a pH of from 6 to 8. The aging is preferably conducted at anelevated temperature, for example a temperature of from 80° to 100° C.,and for a period of time ranging from fifteen minutes to two hours.

Finally, it has been observed that it is possible to further improve thecompatibility of the silica of the invention by another supplementaltreatment.

This treatment entails the use of an alkaline earth metal. This elementmay be introduced either into the suspension or gel of the silica, or,preferably, into the filter cake, in particular after the comminutionthereof, in the form of a salt or hydroxide, for example.

More particularly, a complexing organic salt of an alkaline earth metal,typically a barium salt, for example a barium acetate, is used.

Improved Dentifrice Compositions

This embodiment of the invention also features improved dentifricecompositions containing the above novel silica particulates,advantageously prepared by the aforesaid distinct processes.

The silica particulates of the invention are well adapted forincorporation into dentifrice compositions comprising at least oneelement selected from among the fluorides, phosphates, guanidines, andespecially chlorhexidine. The silica particulates advantageouslydisplays a compatibility, according to the tests described hereinafter,of at least 90% for each of these elements.

The silica particulates of the invention are also compatible with maleicacid/vinylethylether copolymers and may be incorporated in dentifricecompositions containing these copolymers. Finally, they advantageouslyhave a compatibility with zinc of at least 50% and preferably at least80%.

The composition of the improved dentifrice incorporating the novelsilica particulates is as described above in Embodiment I.

Illustrative Examples

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

Measurement of compatibility with chlorhexidine:

4 g silica were dispersed in 16 g of a 1% aqueous solution ofchlorhexidine digluconate.

The suspension was agitated for 24 h at 37° C.

The suspension was then centrifuged at 20,000 rpm or 30 min and thesupernatant was filtered on a 0.2 μm Millipore filter.

0.5 ml of the filtered solution was withdrawn and diluted in 100 mlwater in a volumetric flask. This solution constituted the testsolution.

A reference solution was prepared by the same procedure, but without thesilica. A 1% aqueous solution of chlorhexidine digluconate was agitatedfor 24 h at 37° C., then centrifuged at 20,000 rpm and the supernatantfiltered on a 2 μm Millipore filter. 0.5 ml of the resulting solution isdiluted in 100 ml water in a volumetric flask.

The absorbance of the two solutions was then measured at 254 nm by meansof a spectrophotometer (Uvicon 810/830).

The amount of free chlorhexidine, designated the % Compatibility, wasdetermined by the relationship: ##EQU8##

The measurement of the compatibility of the silica particulates withother dentifrice components (fluorides, zinc and sodium and potassiumpyrophosphates) was carried out as described in Embodiment I.

In said examples to follow, the tests described immediately below werecarried out to measure the compatibility of the silica with variouscompounds.

EXAMPLE 4

This example relates to the preparation of a compatible silica of theabrasive type.

Into a reactor equipped with a temperature and pH control system and aturbine agitation system, 6 1 of deionized water were introduced.

After commencing agitation, the contents of the reactor were heated to85° C.

When this temperature was reached, the following materials weresimultaneously added: 8.5 1 sodium silicate having a silicaconcentration of 120 g/l and a SiO₂ /NaO ratio 3.5, at a flow rate of0.34 l/min, and 13.5 l sulfuric acid having a concentration of 80 g/l.The acid flow rate was adjusted such that the pH of the medium wasmaintained at constant value of 8.0.

After 40 min of addition, the addition of the silicate was discontinuedand the addition of the acid continued until the pH of the reactionmixture was stabilized at 4.

The mixture was then aged for 15 min at this pH and at 85 ° C.

The mixture was then filtered and the moist filter cake was washed withdeionized water until the conductivity of the filtrate was less than 100μS/cm.

The filter cake was then washed with water adjusted to pH 4 by theaddition of acetic acid.

A final wash was carried out with deionized water.

The product was then dried by atomization and ground in a forplex typegrinder to produce a grain size of 10 microns.

The physico/chemical properties of the resulting silica particulateswere as follows:

    ______________________________________                                        (i)      BET surface    100    m.sup.2 /g                                     (ii)     CTAB surface   55     m.sup.2 /g                                     (iii)    oil uptake     120    cm.sup.3 /100 g                                (iv)     pH             6.5                                                   ______________________________________                                    

The chemical analyses of the silica are reported in the following table:

    ______________________________________                                        Ions   SO.sub.4   Al     Fe       Ca   C                                      ______________________________________                                        ppm    100        250    130      300  10                                     ______________________________________                                    

The surface chemistry was quantified by the following parameters:

Ho higher than 3.3

PZC=4

Δν=5 cm⁻¹

Number of OH/nm² =9.

The following table sets forth the compatibilities the silica particleswith the various ingredients of a dentifrice composition.

    ______________________________________                                        In-    Fluoride Pyrophosphate                                                                              Chlorhexidine                                                                          Zinc                                    gredients                                                                            (NaF)    (Na/K)       (digluconate)                                                                          (ZnSO.sub.4)                            ______________________________________                                        % Com- 95       98           75       70                                      patibility                                                                    ______________________________________                                    

EXAMPLE 5

The procedure of Example 4 was repeated to obtain a silica filter cakewhich was washed with water having a pH of 4, by addition of acetic acidthereto.

The cake obtained in this manner was comminuted such as to provide afluid suspension.

0.2 g barium acetate was added under agitation.

The silica was then dried by atomization and ground to produce a meanparticle size of 10 μm.

The physico/chemical properties of the resulting silica particles werethe following:

    ______________________________________                                        (i)      BET surface    100    m.sup.2 /g                                     (ii)     CTAB surface   60     m.sup.2 /g                                     (iii)    oil uptake     110    cm.sup.3 /100 g                                (iv)     pH             6.5                                                   ______________________________________                                    

The chemical analyses of the silica are reported in the following table:

    ______________________________________                                        Ions   SO.sub.4   Al     Fe       Ca   C                                      ______________________________________                                        ppm    100        250    130      300  10                                     ______________________________________                                    

The surface chemistry was quantified by the following parameters:

Ho higher than 3.3

PZC=4.5

Δν=2 cm⁻¹

Number of OH/nm² =8

    ______________________________________                                        In-    Fluoride Pyrophosphate                                                                              Chlorhexidine                                                                          Zinc                                    gredients                                                                            (NaF)    (Na/K)       (digluconate)                                                                          (ZnSO.sub.4)                            ______________________________________                                        % Com- 95       98           90       70                                      patibility                                                                    ______________________________________                                    

EXAMPLE 6

This example relates to the preparation of a silica gel thickener.

Into a reactor equipped with a temperature and pH control system and aturbine agitation system, 14 l Na silicate were introduced, having asilica concentration of 86 g/l and a SiO₂ /Na₂ O ratio of 3.5.

After commencing agitation (100 rpm), 1.45 l of 28% ammonia were addedover 2 min.

0.8 l sulfuric acid having a concentration of 200 g/l was thenintroduced, at a flow ram of 0.2 l/min.

The mixture was then reacted for 5 min at a temperature of 20° C.

5.2 l sulfuric acid having a concentration of 200 g/l were then added,at a rate of 0.2 l/min.

Following the appearance of the gel (visual or by measuring theturbidity), the mixture was aged for 10 min.

The gel was dispersed by agitating the mixture at 400 rpm for 30 min.

The pH of the medium was then reduced to a value of 3.5 by the additionof sulfuric acid (200 g/l) at a rate of 0.2 l/min.

The reaction mixture was permitted to stabilize for 1 h at 60° C.

The gel was prepared by filtering this mixture and washing the resultingfilter cake twice with 20 l deionized water at 60° C. and with 20 lwater at pH 3.

The treatment according to the present embodiment was then carried outby washing with deionized water at 20° C. until a seductivity of 200microsiemens.cm⁻¹ was obtained.

The resulting cake was comminuted to form a homogeneous suspension ofsilica in a concentration of 10%, adjusted by the addition of water.

The silica was dried by atomization using an Anhydro type atomizer. Thesilica was then micronized in a JET Pulverizer grinder to produce agrain size of 1.5 μm.

The physico/chemical properties of the resulting silica particles wereas follows:

    ______________________________________                                        (i)     BET surface      450    m.sup.2 /g                                    (ii)    CTAB surface     350    m.sup.2 /g                                    (iii)   oil uptake       300    cm.sup.3 /100 g                               (iv)    pH               6.8                                                  (v)     Apparent density 0.270                                                (vi)    Refractive index 1.445                                                ______________________________________                                    

The chemical analyses of the resulting silica particles are reported inthe following table:

    ______________________________________                                        Ions   SO.sub.4   Al     Fe       Ca   C                                      ______________________________________                                        ppm    500        200    120      300  10                                     ______________________________________                                    

The surface chemistry was quantified by the following parameters:

Ho higher than 3.3

PZC=3.6

No pyridine adsorption bands.

Compatibilities:

    ______________________________________                                        In-    Fluoride Pyrophosphate                                                                              Chlorhexidine                                                                          Zinc                                    gredients                                                                            (NaF)    (Na/K)       (digluconate)                                                                          (ZnSO.sub.4)                            ______________________________________                                        % Com- 90       95           65       50                                      patibility                                                                    ______________________________________                                    

EXAMPLE 7

This example relates to the production of a silica thickening agent.

Into a reactor equipped with a pH and temperature control system, 6 lwater were introduced and subsequently, under agitation, 150 g sodiumsulfate were added thereto.

The mixture was heated to 60° C. and, simultaneously, 10 l sodiumsilicate (Rm=3.5 and SiO₂ =220 g/l) and sulfuric acid having aconcentration of 80 g/l, were added over 40 min.

The flow rate of the sulfuric acid was adjusted such as to maintain thepH of the reaction medium at a constant 7.8.

The pH of the medium was stabilized at 4.0 by the very rapid addition ofsulfuric acid. It was permitted to age for 15 min at 60° C.

The reaction mixture was filtered at 60° C. and the silica filter cakewas washed with deionized water such as to provide a conductivity of thefiltrate of 900 microsiemens.cm⁻¹. The cake was then washed with waterat pH 4, adjusted by the addition of acetic acid and a final wash wascarried out with deionized water.

The silica cake obtained in this manner was comminuted and, underagitation, 2 g calcium acetate were added thereto.

The silica was dried by atomization and micronized by a JET pulverizerto adjust the particle size to 1.5 μm.

The physico/chemical properties of the resulting silica particles wereas follows:

    ______________________________________                                        (i)      BET surface    320    m.sup.2 /g                                     (ii)     CTAB surface   120    m.sup.2 /g                                     (iii)    oil uptake     250    cm.sup.3 /100 g                                (iv)     pH             6.5                                                   ______________________________________                                    

The chemical analyses of the resulting silica particles are reported inthe following table:

    ______________________________________                                        Ions   SO.sub.4   Al     Fe       Ca   C                                      ______________________________________                                        ppm    100        200    120      300  20                                     ______________________________________                                    

The surface chemistry was quantified by the following parameters:

Ho higher than 3.3

PZC=4.0

Number of OH/nm² =11

No pyridine adsorption bands

    ______________________________________                                        In-    Fluoride Pyrophosphate                                                                              Chlorhexidine                                                                          Zinc                                    gredients                                                                            (NaF)    (Na/K)       (digluconate)                                                                          (ZnSO.sub.4)                            ______________________________________                                        % Com- 90       95           90       80                                      patibility                                                                    ______________________________________                                    

EXAMPLE 8

This example relates to the preparation of abrasive silica particulates.

Into a reactor equipped with a temperature and pH control system and aturbine agitation system, 6 l deionized water were introduced.

After commencing agitation (300 rpm), the contents of the reactor wereheated to 85° C.

When this temperature was attained, 8.5 l sodium silicate having aconcentration in silica of 120 g/l, an SiO₂ /Na₂ O ratio of 3.5 and aflow rate of 0.34 1/min, and 13.5 l sulfuric acid having a concentrationof 80 g/l, were simultaneously added. The flow rate of the acid wasadjusted such that the pH of the medium was maintained at a constant8.0.

After 40 min of addition, the addition of silicate was discontinued andthe mixture was aged for 15 min at 85° C. and pH 8.

The addition of the acid was continued until the pH of the mixture wasstabilized at 4.

The mixture was then aged for 15 min at this pH and at 85° C.

The mixture was then filtered and the moist filter cake washed withdeionized water until the conductivity of the filtrate was 100microsiemens.cm⁻¹.

Two washes were then carded out with water adjusted to pH 4, by additionof acetic acid thereto.

A final wash was conducted with deionized water.

The product was then dried by atomization and ground to a gain size of9.0 microns in a forplex type grinder.

The physico/chemical properties of the resulting silica particles wereas follows:

    ______________________________________                                        (i)      BET surface    60     m.sup.2 /g                                     (ii)     CTAB surface   50     m.sup.2 /g                                     (iii)    oil uptake     120    cm.sup.3 /100 g                                (iv)     pH             6.0                                                   ______________________________________                                    

The chemical analyses of the resulting silica particles are reported inthe following table:

    ______________________________________                                        Ions   SO.sub.4   Al     Fe       Ca   C                                      ______________________________________                                        ppm    100        250    100      200  10                                     ______________________________________                                    

The surface chemistry was quantified by the following parameters:

Ho higher than 3.3

PZC=4.5

No pyridine adsorption bands

Compatibilities

    ______________________________________                                        In-    Fluoride  Pyrophosphate                                                                             Chlorhexidine                                                                          Zinc                                    gredients                                                                            (naF)     (Na/K)      (digluconate)                                                                          (ZnSO.sub.4)                            ______________________________________                                        % Com- 95        98          70       80                                      patibility                                                                    ______________________________________                                    

EXAMPLE 9

This example relates to the preparation of a silica thickening agent.

Into a reactor equipped with a turbine agitation system were introduced5.07 l sodium silicate having a concentration in silica of 120 g/l, anda SiO₂ /Na₂ O ratio of 3.5, and 3.8 l deionized water.

After commencing agitation (300 rpm), the contents of the reactor wereheated to 68° C.

When this temperature was attained, 2.64 l sulfuric acid having aconcentration of 80 g/l were added. The flow rate of the acid was 0.120l/min.

After 22 min of addition, the addition of the acid was discontinued andthe mixture was aged for 10 min (a sudden increase in turbidity wasobserved).

4.2 l sulfuric acid was then added over 35 min.

The temperature was increased to 87° C. and a simultaneous addition ofsodium silicate, at a rate of 30 ml/min, and of sulfuric acid, at a rateof 52 ml/min, was carried out for 30 min.

The temperature was increased to 95° C. and 0.523 l sulfuric acid wasadded over 10 min.

The mixture was then aged for 10 min.

Finally, the pH of the medium was adjusted to 4 by the addition of acid.

The mixture was then filtered and the moist filter cake washed withdeionized water until the conductivity of the filtrate was 100microsiemens.cm⁻¹.

Another wash was carried out using water adjusted to pH 4, by additionof acetic acid thereto.

A final wash was performed with deionized water.

The product was then dried by atomization and micronized in a JETPulverizer type grinder to produce a grain size of 1.2 microns.

The physico/chemical properties of the resulting silica particles wereas follows:

    ______________________________________                                        (i)      BET surface    180    m.sup.2 /g                                     (ii)     CTAB surface   170    m.sup.2 /g                                     (iii)    oil uptake     350    cm.sup.3 /100 g                                (iv)     pH             6.8                                                   ______________________________________                                    

The chemical analyses of the resulting silica particles are reported inthe following table:

    ______________________________________                                        Ions   SO.sub.4   Al     Fe       Ca   C                                      ______________________________________                                        ppm    500        200    120      300  10                                     ______________________________________                                    

The surface chemistry was quantified by the following parameters.

Ho higher than 3.3

PZC=3.6

No pyridine adsorption bands

Compatibilities

    ______________________________________                                        In-    Fluoride  Pyrophosphate                                                                             Chlorhexidine                                                                          Zinc                                    gredients                                                                            (naF)     (Na/K)      (digluconate)                                                                          (ZnSO.sub.4)                            ______________________________________                                        % Com- 90        95          70       60                                      patibility                                                                    ______________________________________                                    

EXAMPLE 10

In this example, the preparation of the silica filter cake used as thestarting material for the silica produced in Examples 11 to 15 isdescribed.

Into a 30 l reactor equipped with a temperature and pH control systemand a Mixel type agitation system, 1.4 l sodium silicate having a SiO₂/Na₂ O ratio of 3.45 and a SiO₂ concentration of 135 g/l, preheated to75° C., were introduced. After commencing agitation (300 rpm), thecontents of the reactor were heated to 85° C.

When this temperature was attained, simultaneously, the same sodiumsilicate was added at a rate of 0.28 l/min, as well as sulfuric acidhaving a concentration of 80 g/l, preheated to 75° C., at a rate of 0.16l/min.

The average pH of the medium during the simultaneous addition was 9.8.

After 47 min of simultaneous addition, the silicate addition wasdiscontinued and the acid addition was continued at the same rate toprovide a pH of 8. At this point, the reaction mixture was heated to 95°C. over 10 min, while continuing the addition of the acid to stabilizethe pH of the reaction mixture at 4.2 at this temperature.

The mixture was then aged for 15 min at this pH and at 95° C.

The mixture was subsequently filtered and the moist filter cake waswashed with deionized water until the conductivity of the filtrate was2,000 μS/cm.

The cake obtained at this point was used as the base for the preparationof the controlled surface chemistry silica of Examples 11 to 15.

EXAMPLE 11

The filter cake produced in Example 10 was used.

The cake was dispersed in deionized water to form a suspension of 100g/l silica and the suspension was then filtered. The operation wasrepeated until a conductivity of the filtrate of 100 microsiemens.cm⁻¹was attained.

The cake was then redispersed in the form of a 150 g/l suspension indeionized water and the pH of the latter was adjusted to 6 by theaddition of acetic acid.

After filtering, a final wash was carried out with deionized water.

The product was then dried by atomization and ground in a forplex typegrinder to produce a grain size of 9.0 μm.

EXAMPLE 12

A cake obtained according to Example 10 was washed in deionized wateruntil a conductivity of the filtrate of 500 microsiemens.cm⁻¹ wasobtained.

The cake was then washed with 10 l water adjusted to pH 4 by theaddition of gluconic acid.

A final wash was effected with deionized water.

The cake was comminuted to produce a homogeneous suspension and 8.5 gbarium acetate were added under agitation (Ba(C₂ H₃ O₂)₂. H₂ O). Themixture was permitted to age for 30 min.

The product was then dried by atomization and ground in a forplex typegrinder to produce a grain size of 9.0 μm.

EXAMPLE 13

The filter cake produced according to the procedure of Example 10 waswashed with deionized water until a conductivity of the filtrate of 500microsiemens.cm⁻¹ was obtained.

The cake was then redispersed in the form of a 150 g/l suspension indeionized water and the pH of the latter was adjusted to 6 by theaddition of acetic acid.

Under agitation, 25 g barium hydroxide Ba(OH)₂.8H₂ O were added and themixture aged for 30 min.

The suspension was then filtered and washed with 5 l water.

The product was dried by atomization and ground on a forplex typegrinder to produce a grain size of 9.0 microns.

EXAMPLE 14

A filter cake produced according to the procedure of Example 10 waswashed in deionized water until a conductivity of the filtrate of 100microsiemens.cm⁻¹ was attained.

The cake was subsequently dispersed in the form of a 150 g/l suspensionin deionized water and the pH of the latter was adjusted to 6.3 by theaddition of acetic acid.

A final wash was carried out with deionized water.

Under agitation, 0.1 g barium acetate, (Ba(C₂ H₃ O₂)₂.H₂ O) was addedand the mixture aged for 30 min.

The product was then dried by atomization and ground on a forplex typegrinder to produce a grain size of 9.0 microns.

EXAMPLE 15

The filter cake of Example 10 was used. The cake was dispersed indeionized water to form a 100 g/l silica suspension and the suspensionwas then filtered. The operation was repeated to produce a conductivityof the filtrate of 200 microsiemens.cm⁻¹.

The product was then dried by atomization and ground on a forplex typegrinder to produce a grain size of 9.0 microns.

The properties of the silica particulates of Examples 11 to 15 arereported in the following table:

    __________________________________________________________________________    Surfaces (m.sup.2 /g)                                                                              Oil       Refractive                                     Example                                                                            BET CTAB                                                                              pH OH/nm.sup.2                                                                        uptake                                                                            PZC                                                                              Ho index                                          __________________________________________________________________________    11   250 50  6.9                                                                              12   102 4.5                                                                              >3.3                                                                             1.460                                          12   250 45  6.8                                                                              10   110 4  >3.3                                                                             1.457                                          13   250 60  7.2                                                                              12   105 3.8                                                                              >3.3                                                                             1.458                                          14   260 55  7.0                                                                              10   105 6  >3.3                                                                             1.457                                          15   250 55  7.5                                                                              12   100 3.6                                                                              >3.3                                                                             1.446                                          __________________________________________________________________________

    ______________________________________                                               Ions (ppm)                                                             Examples SO.sub.4  Al     Fe      Ca   C                                      ______________________________________                                        11       70        300    150     300  20                                     12       150       350    200     350  20                                     13       200       400    200     350  50                                     14       50        230    120     120  20                                     15       200       415    200     355  20                                     ______________________________________                                    

Compatibilities

    ______________________________________                                                Fluoride Pyrophosphate                                                                             Chlorhexidine                                                                          Zinc                                    Examples                                                                              (NaF)    (Na/K)      (digluconate)                                                                          (ZnSO.sub.4)                            ______________________________________                                        11      90       95          75       70                                      12      95       98          80       85                                      13      96       95          70       65                                      14      95       96          98       80                                      15      95       95          70       60                                      ______________________________________                                    

COMPARATIVE EXAMPLE 16

As a comparison, in the following table are reported the measurements ofcompatibility of the commercial silica generally used in dentifriceformulations, together with the physico/chemical properties thereofwherein CH_(x) =chlorhexidine, F=fluorine, Zn=zinc andPYR=pyrophosphate.

The measurements were performed by the tests described above.

    __________________________________________________________________________    Silica (trademark                                                                      Surfaces (m.sup.2 /g)                                                                         SO.sub.4 (%                                                                         Compatibilities                                of manufacturer)                                                                       CTAB                                                                              BET Ho OH/nm.sup.2                                                                        by weight)                                                                          CH.sub.x                                                                         F Zn                                                                              PYR                                     __________________________________________________________________________    Hubert   50  100 <3 30   0.65  1  95                                                                              0 95                                      Zeodent 113                                                                   Zeofinn Z113                                                                           70  175 <3 17   0.50  1  96                                                                              0 95                                      Grace    240 400 <3 17   1.56  0  90                                                                              40                                                                              95                                      Syloblanc 81                                                                  Grace        400 <3 17   0.7   4  90                                                                              2 90                                      Syloid 244                                                                    Degussa  180 190 <3 16   1.0   0  90                                                                              20                                                                              90                                      Sipernat 22S                                                                  Rhone Poulenc                                                                          50  250 <3 30   0.8   0  60                                                                              0 90                                      Tixosil 53                                                                    __________________________________________________________________________

It should be noted that the pH was less than 3 for all materialsreported in the table.

EXAMPLE 17

This example relates to the formulation of a translucid, gel typedentifrice composition incorporating the silica particulates of theinvention.

The formula was as follows:

    ______________________________________                                        (i)      Sorbitol (70% aqueous)                                                                             65.00                                           (ii)     Glycerin             0.00                                            (iii)    CMC 7mFD             0.80                                            (iv)     Sodium saccharinate  0.20                                            (v)      Sodium fluoride      0.24                                            (vi)     Sodium benzoate      0.08                                            (vii)    Perfume              2.00                                            (viii)   Abrasive silica, Example 5                                                                         15.00                                           (ix)     Thickening silica, Example 9                                                                       8.00                                            (x)      Chlorhexidine digluconate                                                                          1.00                                            (xi)     Distilled water      7.68                                            ______________________________________                                    

The dentifrice had satisfactory rheological properties and extrudedsuitably, both initially and after storage (2 months).

Visual examination of the dentifrice confirmed that the strength of theextrusion was proper and that it was present in the form of a translucidgel.

This dentifrice had the following properties:

    ______________________________________                                        pH, dilution at 10%:        6.8                                               Abrasive strength on copper, LNE standard (mg):                                                           5.6                                               Plastic viscosity (Pa · s):                                                                      0.5                                               ______________________________________                                    

Anti-bacterial activity was present.

EXAMPLE 18

This example relates to the formulation of an opaque paste typedentifrice:

The formula was as follows:

    ______________________________________                                        (i)    Glycerin:                22.00                                         (ii)   CMC 7MFD:                1.0                                           (iii)  Sodium saccharinate:     0.20                                          (iv)   Sodium monofluorophosphate:                                                                            0.76                                          (v)    Sodium fluoride:         0.10                                          (vi)   Sodium lauryl sulfate (30% aqueous):                                                                   4.67                                          (vii)  Sodium benzoate:         0.10                                          (viii) Perfume:                 0.90                                          (ix)   Titanium dioxide:        1.00                                          (x)    Abrasive silica, Example 8:                                                                            31.50                                         (xi)   ZnSO.sub.4.7H.sub.2 O:   0.48                                          (xii)  Distilled water:         37.29                                         ______________________________________                                    

Rheological and visual examination of the resulting dentifrice evidencedthat the usual properties of the dentifrice were good.

Embodiment III

In a third embodiment of the present invention, the novel silicaparticulates are improvedly compatible with the aforementioned organicamino compounds and particularly the above described class offluorine-containing amines and betaines. These silica particulates arecharacterized in that the pH of an aqueous suspension thereof varies asa function of its concentration and its electrical conductivity inaccordance with the equations given above.

As indicated above, the essential characteristics of the silicaparticulates of the invention reside in the surface chemistry thereof.More specifically, one of the aspects to be taken into account in thissurface chemistry is the acidity. The term "acidity" is used in thesense described in Embodiment II, above. More particularly, the acidityfunction Ho is used to characterize the silica particulates of thepresent embodiment. Ho is determined in the manner described above inEmbodiment II. The protocol for measuring the pH as a function of theconcentration of the aqueous silica suspension and its electricalconductivity has been described above in Embodiment I.

The surface state of the silica according to the invention is such thatconditions regarding the number of acid surface sites are observed. Thisnumber may be measured as the number of OH⁻ or silanol groups per nm²,the procedure of which was described Embodiment I.

In the present case, the silica particulates of the present aspect ofthe invention advantageously have a number of OH⁻ /nm² equal to or lessthan 12, preferably at most 10, and more particularly ranging from 6 to10.

The nature of the OH⁻ sites of the silica particulates of the invention,which is also a characteristic of their surface chemistry, too may beevaluated by the zero charge point. The point of zero charge is definedby the pH of a silica suspension for which the electric charge of thesurface of the solids is zero, regardless of the ionic strength of themedium. This (ZCP) measures the real pH of the surface, to the extentthat it is free from all ionic impurities. The (ZCP) is determined asdescribed in Embodiment I.

In practice, it is preferred that the (ZCP) be at least 4,advantageously ranging from 4 to 6. For a better compatibility with themetal cations, it is at most 6.5. For good compatibility with fluorinevalues, the (ZCP) is preferably at most 7.

Moreover, in order to improve the compatibility of the silicaparticulates according to this embodiment of the invention with respectto other constituents and in particular fluorine, the content ofdivalent and higher valency cations contained in the silica is at mostequal to 1,000 ppm. It is particularly desirable that the aluminumcontent of the silica particulates of the invention be at most 500 ppm.Moreover, the iron content of the silica particulates of the inventionis advantageously at most 200 ppm. Preferably, the calcium content is atmost 500 ppm and more preferably at most 300 ppm.

The silicas according to the present embodiment preferably also have acarbon content of at most 50 ppm and particularly at most 10 ppm.

The silica particulates according to the present embodiment, which arecompatible with organic amino compounds, are also compatible with thedifferent metal cations contained in dentifrice compositions. Thus, thelatter can comprise, inter alia, metal cations having a valency greaterthan 1 and which are provided by active molecules. For example,representative thereof are the divalent and higher valency metal cationsof Groups IIa, IIIa, IVa and VIII of the Periodic Table. Particularlyexemplary are the cations of Group Ha, namely, calcium, strontium andbarium, cations of Group IIIa, namely, aluminum and indium, of GroupIVa, namely, germanium, tin and lead and of Group VIII, namely,manganese, iron, nickel, zinc, titanium, zirconium, palladium, etc.

Such cations may be in the form of mineral salts thereof, e.g.,chloride, fluoride, nitrate, phosphate or sulfate, or in the form oforganic salts, e.g., acetate, citrate, etc. More specific examplesinclude zinc citrate, zinc sulfate, strontium chloride, tin fluoride inthe form of the single salt (SnF₂) or in the form of the double salt(SnF₂ /KF), stannous chlorofluoride SnClF and zinc fluoride (ZnF₂).

The silica particulates, according to this embodiment of the invention,are compatible with the different metal cations. The compatibility ofthe subject silicas with such cations, as determined by the tests givenbelow, is at least approximately 50%, preferably at least 70% and morepreferably at least 80%.

These silica particulates can consequently be formulated with advantageinto dentifrice compositions containing divalent and higher valencycations and more particularly in compositions incorporating at least oneof the following components: zinc citrate, zinc sulfate, strontiumchloride and tin fluoride.

In a preferred embodiment of the invention, the novel silicaparticulates are also compatible with guanidines and in particularchlorhexidine. The compatibility, measured by the tests given below, isat least approximately 30%. It can be improved to at least 60% andpreferably to at least 90%.

In this case, the silica has a content of anions of the type SO₄ ²⁻,Cl⁻, NO₃ ⁻, PO₄ ³⁻, CO₃ ²⁻ of at least 5.10⁻³ mole/100 g of silica. Thelower this content, the higher will be the compatibility. In a preferredembodiment, it is at most 1.10⁻³ moles and more particularly 0.2.10⁻³moles/100 g of silica.

In the case of silicas prepared from sulfuric acid, such anion contentis more appropriately expressed by a content in SO₄ ⁼ and by weight. Inthis event, the content is at the most 0.1%. In another preferredembodiment of the invention, such content is at most 0.05% and moreparticularly at most 0.01%.

Thus, the silica particulates according to the invention areparticularly well suited for use in dentifrice compositions containingguanidines and bisguanides. Such compositions are described in U.S. Pat.Nos. 3,934,002 and 4,110,083, hereby incorporated by reference.

The pH of the silica particulates according to the invention, measuredaccording to standard NFT 45-007, is generally at most 8 and preferablyit ranges from 6.0 to 7.5.

The above characteristics provide a silica compatible with theaforementioned organic amino compounds, metal cations and, dependingupon the particular case, also with fluorides and guanidines, inparticular chlorhexidine.

In addition to the surface chemistry characteristics described above,which impart compatibility thereto, the silica particulates of thisaspect of the invention also have physical properties which areperfectly suited for their use in a dentifrice. Thesephysical/structural properties have been more fully described above.

Process for the Preparation of Novel Silica Particulates

The process for the preparation of the silica particulates of thisaspect of the invention will now be described in greater detail. Asindicated above, this process is of the type comprising reacting asilicate with an acid, resulting in the formation of a silica suspensionor a silica gel.

It will be appreciated that any known operation may be used to preparethis suspension or gel (addition of acid to a silicate sediment,simultaneous total or partial addition of acid and silicate to a watersediment, or silicate solution, etc.), with the selection being madeessentially as a function of the physical characteristics of the silicawhich is sought to be produced.

In this embodiment, preferably, the silica gel or suspension is preparedby simultaneously adding the silicate and the acid to a sediment, whichcan be a water sediment, a colloidal silica dispersion containing 0 to150 g/l of silica, expressed as SiO₂, a silicate or an inorganic ororganic salt, preferably of alkali metals, such as, e.g., sodium sulfateor sodium acetate. The addition of these two reagents is carried outsimultaneously in such manner that the pH is maintained constant at avalue of from 4 to 10, preferably from 8.5 to 9.5. The temperatureadvantageously ranges from 60° to 95° C.

One technique for preparing the colloidal silica dispersion, preferablyhaving a concentration of from 20 to 150 g/l entails heating an aqueoussilicate solution, e.g., at a temperature of from 60° to 95° C., andadding the acid to said aqueous solution until a pH is obtained rangingfrom 8.0 to 10.0 and preferably close to 9.5.

The concentration of the aqueous silicate solution, expressed as SiO₂,preferably ranges from 20 to 150 g/l. It is possible to use a diluted orconcentrated acid, and its normality can range from 0.5 to 36N,preferably from 1 to 2N.

The silicate is advantageously an alkali metal silicate and preferably asodium silicate, with a SiO₂ /Na₂ O weight ratio of from 2 to 4 andpreferably equal to 3.5. The acid can be gaseous, such as carbon dioxidegas, or liquid, preferably sulfuric acid.

In a further stage of the process of this embodiment, the suspension orgel is subjected to a plural aging operation. A first aging is cardedout at a pH of at most 8.5 and, e.g., ranging from 6 to 8.5 andpreferably at 8.0. Aging is preferably carried out hot, e.g., at atemperature of from 60° to 100° C. and preferably at 95° C. for a periodof time ranging from 10 minutes to 2 hours.

A variant of this embodiment comprises preparing a silica gel or asilica suspension by progressively adding the acid to a sedimentcontaining the silicate, until the desired aging pH is attained. Thisoperation is carried out at a temperature preferably ranging from 60° to95° C. The suspension of the silica gel is then aged under theconditions described hereinbefore.

This is followed by a second aging at a pH below 6, preferably rangingfrom 5 to 6 and even more preferably is equal to 5.5. The temperatureand time conditions are the same as for the first aging step. Acid isadded to attain the desired aging pH.

It is also possible to use an inorganic acid such as nitric,hydrochloric, sulfuric or phosphoric acid, or even carbonic acid formedby bubbling carbon dioxide gas.

A third aging step is then carried out, at a pH below 5, preferably from3 to 5, and even more preferably about 4.

The temperature and time conditions are the same as for the two otheraging operations. Acid is added to obtain the desired aging pH. Thesilica is then separated from the reaction medium by any known means,such as, e.g., a vacuum filter or a filter press. Thus, a silica cake isrecovered.

The process according to this embodiment can then proceed according totwo principal variants.

The first variant relates to the preparation of silicas compatible withthe organic amino compounds and the divalent and higher valency metalcations. In this case, the process entails washing the cake underconditions such that the pH of the suspension or the medium beforedrying must comply with the following equation:

    pH=d-e log (D)                                             (III)

in which e is a constant equal to or greater than 0.6 and equal to orless than 1.0; d is a constant equal to or greater than 7.0 and equal toor less than 8.5; and (D) represents the electrical conductivity of theaqueous silica suspension, expressed in microsiemens.cm⁻¹.

The washing is advantageously with water, preferably deionized water,and/or using an acid solution having a pH of from 2 to 7.

This acid solution may be, for example, a solution of an inorganic acidsuch as nitric acid.

However, preferably, said acid solution can also be a solution of anorganic acid, particularly a complexing organic acid. This acid can beselected from among carboxylic, dicarboxylic, hydroxycarboxylic andaminocarboxylic acids. Examples of such acids are acetic acid andexamples of the complexing acids are tartaric, maleic, glyceric,gluconic and citric acids.

Particularly when using a solution of an inorganic acid, it can beadvantageous to carry out a final washing with deionized water.

The second variant relates to the preparation of silicas which are alsocompatible with guanidines and, in particular, chlorhexidine.

In this case, a more pronounced washing is carried out. It must becontinued until a washing filtrate is obtained, whose conductivity is atthe most 200 microsiemens-cm⁻¹ and preferably below 100microsiemens.cm⁻¹. As indicated above, it is important that the anionconcentration is at most 5.10⁻³ mole/100 g of silica.

As a function of the particular case, it is possible to carry out one ormore washing operations, typically two such washing with water andpreferably deionized water and/or with an aqueous solution of an organicacid, particularly those indicated above.

From a practical standpoint, the washing operations can be carried outby pouring the washing solution onto the cake, or by introducing thelatter into the suspension obtained, following the crumbling of thecake. Thus, the filter cake, prior to the drying operation, is subjectedto crumbling or disintegration, which can be carried out by any knownmeans, e.g., a high speed stirrer.

Thus, before or after washing, the silica cake is comminuted and thendried by any known means. Drying can be carried out in a tunnel ormuffle furnace, for example, or by atomization in a hot air stream, theinlet temperature of which can range from approximately 200° to 500° C.and whose outlet temperature ranges from approximately 80° to 100° C.The residence time advantageously ranges from 10 seconds to 5 minutes.

If necessary, the dried material can be ground to provide the desiredgrain or particle size. This operation is carried out in a conventionalapparatus, such as an impeller mill or an air jet grinder.

Improved Dentifrice Compositions

The present embodiment of the invention also features novel dentifricecompositions containing the silicas described above, or prepared by theprocess also described above.

The amount of such silica incorporated in the dentifrice compositions ofthe present invention can vary over wide limits, but typically rangesfrom 5% to 35% by weight.

The silicas according to the present embodiment can be used moreparticularly in dentifrice compositions containing at least oneconstituent selected from among the fluorides, phosphates, guanidinesand in particular chlorhexidine. Thus, they can have a compatibility,according to the tests given below, of at least 90% for each of theseconstituents.

They are also suitable for dentifrice compositions containing at leastone constituent selected from among the organic amino compounds andcompounds supplying a divalent and higher valency metal cation. They canthen have a compatibility, also according to the tests given below, ofup to 80% for each constituent.

The silicas according to this aspect of the invention are particularlywell suited for dentifrice formulations simultaneously containing atleast one inorganic fluoride and/or organic fluoride, at least one alkylbetaine and at least one guanidine, in particular chlorhexidine.

Preferred such formulations include a sodium fluoride and/or a tinfluoride and/or a fluorine-containing amine, more specifically cetylamine hydrofluoride,bis-(hydroxyethyl)-aminopropyl-N-hydroxyethyl-octadecyl aminedihydrofluoride, an alkyl betaine and chlorhexidine.

The silicas according to the present embodiment are also compatible withmaleic acid/vinyl ethyl ether copolymers and, therefore, can also beincorporated into dentifrice compositions comprising these copolymers.

The composition of a dentifrice including the novel silica particulatesis more fully described above in Embodiment I.

Illustrative Examples

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

In said examples to follow, the pH measuring protocol as a function ofthe conductivity and concentration, as well as the tests for measuringthe compatibility of the silica with the various constituents, werecarried out according to the following techniques:

pH Measurement protocol as a function of the silica concentration andits conductivity:

Silica suspensions having increasing concentrations ranging from 0% to25% by weight were formed by dispersing a mass 100 m of degassed,deionized water (Millipore quality). The suspensions were stirred for 24hours at 25° C.

The pH of the suspensions and solutions obtained after centrifuging afraction of the suspension at 8,000 r.p.m. for 40 min, and filtering ona 0.22 μm Millipore filter, were measured at 25° C. under a nitrogenatmosphere using a Titroprocessor Metrohm 672-type measuring system.

In the same manner, the conductivity of the resulting suspensions andsolutions was measured at 25° C. using a Radiometer conductivity meter(CDM83) equipped with a CDC304 cell with a cell constant equal to 1cm⁻¹. The conductivity is reported in microsiemens/cm (μS/cm).

The suspension effect (SE) is defined as the difference in pH betweenthe pH of a 20% silica suspension and the pH of its supernatant solutionseparated by centrifugation.

Measurement Of compatibility withbis-(hydroxyethyl)-aminopropyl-N-hydroxyethyl-octadecyl aminedihydrofluoride;

(1) An aqueous solution containing 1.65% of fluorine-containing aminewas formed by adding 5 g of 33% fluorine-containing amine to propanediolin 95 g of twice distilled water.

(2) 4 g of silica were dispersed in 16 g of the solution produced in(1). The thus obtained suspension was stirred for 4 weeks at 37° C.

(3) The suspension was then centrifuged at 8,000 r.p.m. for 30 min andthe supernatant obtained was filtered on a 0.22 μm Millipore filter.

(4) The free fluorine-containing amine concentration was determined bynitrogen microanalysis of the solution obtained in (1) and of thesupernatant obtained in (3).

(5) The compatibility was determined by the following relationship:##EQU9##

Hereinafter, the percentage fluorine-containing amine compatibility isdesignated AF.

Measurement of compatibility with cetyl amine hydrofluoride:

(1) An aqueous solution containing 1.72% of fluorine-containing aminewas formed by dissolving 1.72 g of cetyl amine hydrofluoride in 98.28 gof twice distilled water.

(2) 4 g of silica were dispersed in 16 g of the solution produced in(1). The thus obtained suspension was stirred for 4 weeks at 37° C.

(3) The suspension was then centrifuged at 8,000 r.p.m. for 30 min andthe supernatant obtained was filtered on an 0.22 μm Millipore filter.

(4) The free fluorine-containing amine concentration was determined bynitrogen microanalysis of the solution obtained in (1) and of thesupernatant obtained in (3).

(5) The compatibility was determined by the following relationship:##EQU10##

Hereinafter, the percentage fluorine-containing amine compatibility isdesignated AFc.

Measurement of compatibility with an alkyl betaine:

The alkyl betaine used was the product marketed by AKZO under thetrademark ARMOTERIC LB.

(1) An aqueous solution containing 2% alkyl betaine was formed bydissolving 6.67 g of 30% alkyl betaine in 93.33 g of twice distilledwater.

(2) 4 g of silica were dispersed in 16 g of the solution produced in(1). The thus obtained suspension was stirred for 4 weeks at 37° C.

(3) The suspension was then centrifuged at 8,000 r.p.m. for 30 min andthe supernatant obtained was filtered on an 0.22 μm Millipore filter.

(4) The free alkyl betaine concentration was determined by organiccarbon microanalysis of the solution obtained in (1) and the supernatantobtained in (3).

(5) The compatibility was determined by the following relationship:##EQU11##

Hereinafter, the percentage alkyl betaine compatibility is designedaBeta.

Measurement Of compatibility with an alkylamidoalkyl betaine:

(1) An aqueous solution containing 2.0% alkylamidoalkyl betaine wasformed by dissolving 6.67 g of 30% alkylamidoalkyl betaine in 93.33 g oftwice distilled water.

(2) 4 g of silica were dispersed in 16 g of the solution produced in(1). The thus obtained suspension was stirred for 4 weeks at 37° C.

(3) The suspension was then centrifuged at 8,000 r.p.m. for 30 min andthe supernatant obtained was filtered on an 0.22 μm Millipore filter.

(4) The free alkylamidoalkyl betaine concentration was determined byorganic carbon microanalysis of the solution obtained in (1) and thesupernatant obtained in (3).

(5) The compatibility was determined by the following relationship:##EQU12##

Hereinafter, the percentage alkylamidoalkyl betaine compatibility isdesignated by Beta.

Measurement of compatibility width chlorhexidine:

4 g of silica were dispersed in 16 g of an aqueous chlorhexidinesolution having a 1% concentration in chlorhexidine digluconate. Thesuspension was stirred for 24 hours at 37° C. The suspension was thencentrifuged at 20,000 r.p.m. for 30 min and the supernatant obtained wasfiltered on an 0.2 μm Millipore filter. 0.5 ml of the thus filteredsolution was sampled and diluted in 100 ml of water in a graduatedflask. This solution formed the test solution.

A reference solution was formed using the same protocol, but without thesilica. A 1% chlorhexidine digluconate aqueous solution was stirred for24 hours at 37° C., centrifuged at 20,000 r.p.m. and the supernatantfiltered on an 0.2 μm Millipore filter. 0.5 ml of the thus obtainedsolution was diluted in 100 ml of water in a graduated flask.

The absorptivity of the two solutions was then measured at 254 nm usinga spectrophotometer (Uvikon 810/820).

The free chlorhexidine quantity (CH_(x)), designated % compatibility,was determined by the relationship: ##EQU13##

Measurement of compatibility with zinc, fluorides, and sodium andpotassium pyrophosphates was as described above.

EXAMPLE 19

Into a reactor equipped with a temperature and pH regulating system anda propeller stirring system (Mixel), were introduced 8.32 liters ofsodium silicate having a 130 g/l silica concentration and a SiO₂ /Na₂ Omolar ratio of 3.5 and 8.33 liters of soft water having a conductivityof 1 μS/cm. After beginning stirring (350 r.p.m.), the thus formedsediment was heated at 9020 C. When the desired temperature wasattained, sulfuric acid having a concentration of 80 g/l was added at aconstant flow rate of 0.40 l/min in order to adjust the pH to 9.5.

This was followed by the simultaneous addition of 45.25 l of sodiumsilicate having a silica concentration of 130 g/l, a SiO₂ /Na₂ O molarratio of 3.5 and a flow rate of 0.754 l/min and 29.64 l of 80 g/lsulfuric acid. The sulfuric acid rate was adjusted such as to maintainthe pH of the reaction medium at a constant value of 9.5.

After adding for 60 min, the sodium silicate addition was terminated andsulfuric acid addition continued at a rate of 0.494 l/min until the pHof the reaction mixture was stabilized at 8. During this phase, thetemperature of the medium was increased to 95° C. This was followed byaging for 30 min at said pH and at 95° C. During aging, the pH wasmaintained at 8 by adding acid.

Upon completion of aging, the pH was adjusted to 5.5 by adding sulfuricacid at a flow rate of 0.494 l/min and this was followed by aging for 30min at said pH and at 95° C. Upon completion of aging, the pH wasadjusted to 3.5 by adding sulfuric acid and was maintained at this levelfor 30 min.

After discontinuing the heating, the mixture was filtered and the cakeobtained washed with deionized water until a filtrate having aconductivity of 2000 μS/cm was obtained. The cake obtained after washingwas dispersed in the presence of deionized water to form a suspensionhaving a silica concentration of 10%. The pH of the suspension wasadjusted to 6 by adding acetic acid.

Thereafter, a second filtration was carried out, and then a waterwashing, such as to adjust the conductivity to 500 μS/cm and waterwashing with a pH adjusted to 5 by acetic acid, such as to adjust the pHto 5.5. The medium was then monitored to ensure that the followingrelationship existed:

    pH≦8.20-0.91 log (D).

The cake was then crumbled and the silica dried by atomization.Thereafter, the silica obtained was ground on an impeller mill in orderto produce a powder, the average agglomerate diameter of which, measuredon a COULTER counter, was 8 μm.

The physico/chemical characteristics of the thus obtained silica areindicated in the following table:

    ______________________________________                                        BET surface, m.sup.2 /g 65                                                    CTAB surface, m.sup.2 /g                                                                              60                                                    DOP oil absorption, ml/100 g of silica                                                                125                                                   Pore volume, Hg, cm.sup.3 /g                                                                          1.90                                                  pH (5% water)           6.2                                                   Refractive index        1.450                                                 Translucency %          90                                                    Na.sup.+  ppm           60                                                    SO.sub.4.sup.=  ppm     100                                                   Al.sup.3+  ppm          200                                                   Fe.sup.3+  ppm          120                                                   Ca.sup.2+  ppm          30                                                    Cl.sup.-  ppm           20                                                    C ppm                   5                                                     ______________________________________                                    

Table II below reports the surface chemistry characteristics of thesilica according to the invention, as well as the results of thecompatibility with the organic amino compounds. Table III reports thecompatibility results with the conventional components of dentifriceformulations, namely, chlorhexidine, fluoride, zinc and pyrophosphate.

EXAMPLE 20

Into a reactor equipped with a temperature and pH regulating system anda propeller stirring system (Mixel) were introduced 530 l of sodiumsilicate having a silica concentration of 135 g/l and a SiO₂ /Na₂ Omolar ratio of 3.5 and 15 l of soft water having a conductivity of 1μS/cm. After actuating the stirring system (350 r.p.m.), the thus formedsediment was heated to 90° C. When this temperature was attained,sulfuric acid having a concentration of 80 g/l was added at a constantflow rate of 0.38 l/min to adjust the pH to 9.5.

This was followed by the simultaneous addition of 44.70 l of sodiumsilicate having a 135 g/l silica concentration, a SiO₂ /Na₂ O molarratio of 3.5 and a flow rate of 0.745 l/min and 25.30 l of 80 g/lsulfuric acid. The sulfuric acid rate was adjusted such as to maintainthe pH of the reaction medium at a constant value of 9.5.

After 60 min of addition, the sodium silicate addition was terminatedand sulfuric acid addition was continued at a rate of 0.350 l/min untilthe pH of the reaction mixture was stabilized at 8. During this phase,the temperature of the medium was increased to 95° C. This was followedby aging for 30 min at this pH and at 95° C. During aging the pH wasmaintained at 8 by acid addition. Upon completion of aging the pH wasadjusted to 5 by adding sulfuric acid at a rate of 0.400 l/min andsubsequently another aging was carried out for 30 min at said pH and at95° C. Upon completion of this aging, the pH was adjusted to 3.5 byadding sulfuric acid and said pH was maintained at 3.5 for 30 min.

After discontinuing heating, the mixture was filtered and the cakeobtained washed with deionized water until a filtrate was obtainedhaving a conductivity of 2000 μS/cm. The cake was then crumbled in thepresence of water to form a 20% silica suspension and the pH wasadjusted to 5.1 such as to ensure that the following relationshipexisted:

    pH≦8.20-0.91 log (D).

The silica was dried by atomization and ground on an impeller mill toproduce a powder, the mean agglomerate diameter of which was 8 μm.

The physico/chemical characteristics of the thus obtained silica areindicated below:

    ______________________________________                                        BET surface, m.sup.2 /g 100                                                   CTAB surface, m.sup.2 /g                                                                              80                                                    DOP oil absorption, ml/100 g of silica                                                                200                                                   Pore volume, Hg, cm.sup.3 /g                                                                          3.35                                                  pH (5% water)           6.5                                                   Refractive index        1.455                                                 Translucency %          95                                                    SO.sub.4.sup.=  %       0.5                                                   Na.sup.+  %             0.25                                                  Al.sup.3+  ppm          350                                                   Fe.sup.3+  ppm          120                                                   Ca.sup.2+  ppm          50                                                    Cl.sup.-  ppm           20                                                    C ppm                   5                                                     ______________________________________                                    

Table II below reports the surface chemistry characteristics of thesilica according to the invention, as well as the compatibility resultswith the organic amino compounds. Table III reports the compatibilityresults with the conventional components of dentifrice formulations,namely chlorhexidine, fluoride, zinc and pyrophosphate.

EXAMPLE 21

Into a reactor equipped with a temperature and pH regulating system anda propeller stirring system (Mixel) were introduced 5.60 l of sodiumsilicate having a silica concentration of 135 g/l and a SiO₂ /Na₂ Omolar ratio of 3.5.

After actuating the stirring system (350 r.p.m.), the thus formedsediment was heated to 85° C. When this temperature was attained,sulfuric acid having a concentration of 85 g/l and preheated to 70° C.was added at a constant flow rate of 0.50 l/min in order to adjust thepH to 9.7.

This was followed by the simultaneous addition of 52.64 l of sodiumsilicate having a 135 g/l silica concentration, a molar ratio SiO₂ /Na₂O of 3.5 and at a rate of 0.745 l/min and 30 l of 85 g/l sulfuric acid.The sulfuric acid rate was adjusted such as to maintain the pH of thereaction medium at a constant value of 9.7. The simultaneous additionwas carried out at 85° C. using reagents preheated to 70° C.

After continuing the additions for 45 min, sodium silicate addition wasterminated and sulfuric acid addition continued at a rate of 0.450 l/minuntil the pH of the reaction mixture was stabilized at 8. During thisphase, the temperature of the medium was increased to 95° C. This wasfollowed by aging for 10 min at said pH and 95° C. During aging, the pHwas maintained at 8 by adding acid. Upon completion of aging the pH wasadjusted to 5 by adding sulfuric acid at a rate of 0.750 l/min and thiswas followed by a second aging for 15 min at said pH and 95° C. Uponcompletion of this aging, the pH was adjusted to 3.7 by adding sulfuricacid and said pH level was maintained for 60 min.

After discontinuing heating the mixture was filtered and the cakeobtained washed with deionized water until a filtrate having aconductivity of 2500 μS/cm was obtained. The cake was then crumbled inthe presence of water to form a 20% silica suspension and the pH wasadjusted to 5.5 to ensure that the following relationship existed:

    pH≦7.5-0.70 log (D).

The silica was dried by atomization and ground on an impeller mill toproduce a powder having a mean agglomerate diameter of 8 μm.

The physico/chemical characteristics of the thus obtained silica areindicated in the following table:

    ______________________________________                                        BET surface, m.sup.2 /g 200                                                   CTAB surface, m.sup.2 /g                                                                              55                                                    DOP oil absorption, ml/100 g of silica                                                                110                                                   Pore volume, Hg, cm.sup.3 /g                                                                          2.65                                                  pH (5% water)           7.0                                                   Refractive index        1.460                                                 Translucency %          85                                                    SO.sub.4.sup.=  %       200                                                   Na.sup.+  %             60                                                    Al.sup.3+  ppm          150                                                   Fe.sup.3+  ppm          120                                                   Ca.sup.2+  ppm          50                                                    Cl.sup.-  ppm           20                                                    C ppm                   5                                                     ______________________________________                                    

The following Table II reports the surface chemistry characteristics ofthe silicas of the invention described in Examples 19 to 21. It alsoreports the compatibility results of the silicas according to theinvention with organic amino compounds.

Table III reports the compatibility results with the conventionalcomponents used in dentifrice formulations, namely, chlorhexidine,fluoride, zinc and pyrophosphate.

For comparison purposes, Tables II and III provide the characteristicsand different compatibilities of commercially available silicas, thefollowing list constituting a representative range thereof:

S81: Syloblanc 81 (GRACE)

Z113: Zeodent 113 (HUBER)

Sid12: Sident 12 (DEGUSSA)

Sy115: Sylox 15 (GRACE)

T73: Tixosil 73 (RHONE-POULENC)

T83: Tixosil 83 (RHONE-POULENC).

                                      TABLE II                                    __________________________________________________________________________    Physicochemical characteristics and compatibility with organic                amino compounds of silicas according to the invention and conventional        silicas                                                                       Physicochemical characteristics of silicas                                                                % compatibility                                   Silica                                                                              pH/log (C)                                                                          pH/log (D)                                                                          SE  Ho ZPC                                                                              AF Beta                                                                             CHx                                         __________________________________________________________________________    S81   4.7-0.75x                                                                           7.0-0.62z                                                                           -0.17                                                                             ≦2                                                                        2.2                                                                              0  0  0                                           Z113  8.1-0.94x                                                                           10-1.0z                                                                             -0.70                                                                             ≦3                                                                        2.5                                                                              0  0  0                                           Sid12 7.6-0.55x                                                                           8.5-0.60z                                                                           -0.20                                                                             ≦3                                                                        2.8                                                                              0  0  0                                           Sy115 8.1-0.70x                                                                           9.2-0.74z                                                                           -0.94                                                                             ≦3                                                                        2.5                                                                              0  0  0                                           T73   8.6-0.81x                                                                            10-0.87z                                                                           -0.20                                                                             ≦3                                                                        3.0                                                                              0  0  0                                           T83   7.5-0.60x                                                                           8.6-0.60z                                                                           -0.18                                                                             ≦3                                                                        2.5                                                                              0  0  0                                           Example 19                                                                          7.5-0.30x                                                                           8.0-0.50z                                                                           -0.00                                                                             ≧4                                                                        4.2                                                                              85 60 95                                          Example 20                                                                          6.5-0.80x                                                                           8.2-0.90z                                                                           -0.10                                                                             ≧4                                                                        4.5                                                                              85 50 30                                          Example 21                                                                          7.0-0.40x                                                                           7.4-0.60z                                                                           -0.06                                                                             ≧4                                                                        4.0                                                                              80 50 90                                          __________________________________________________________________________

The symbols used in the above Table have the following definitions:pH/log(C) represents the equation pH=b-a·log(C), in which a and b aretwo constants and C is the weight percentage of silica in thesuspension; pH/log(D) represents the equation pH=b'-a'log(D), in whichb' and a' are two constants and D is the conductivity of the silicasuspension in μS/cm; SE represents the suspension effect measured by therelation SE=pH suspension-pH supernatant defined elsewhere; Ho is theHammett constant; ZCP represents the pH for which the surface charge ofthe silica is zero; AF, Beta, and CHx represent the compatibilitypercentages of fluorine-containing amines, alkyl betaine andchlorhexidine respectively, such amounts being indicated above. Thecompatibility percentages obtained with fluorine-containing amine AFcand the alkyl betaine aBeta were similar to those obtained, respectivelyfor AF and Beta.

                  TABLE III                                                       ______________________________________                                        Compatibility of silicas with the active molecules:                           % Compatibility                                                               Silica  P.sub.2 O.sub.7.sup.=                                                                  Zn.sup.++                                                                              F.sup.-                                                                            AF    Beta  CHx                                ______________________________________                                        S81     80       0        90   0     0     0                                  Z113    90       0        95   0     0     0                                  Sid12   80       10       90   0     0     0                                  Sy115   80       0        90   0     0     0                                  T73     90       20       90   0     0     0                                  T83     95       10       95   0     0     0                                  Example 19                                                                            95       80       95   85    60    95                                 Example 20                                                                            90       75       95   85    50    30                                 Example 21                                                                            95       80       95   80    50    90                                 ______________________________________                                    

The results of this Table evidence that the silicas according to theinvention, more particularly compatible with organic amino compounds,differ markedly compared with the standard silicas in consideration ofthe following relationships:

    pH≦8.5-0.40 log (D) and pH≧7.0-0.60 log (D)

    pH≦7.5-0.70 log (C) and pH≧5.0-0.50 log (C)

Embodiment IV

A fourth embodiment of the present invention features novel silicaparticulates especially well adapted for compatibility with metalcations, in particular zinc, tin, strontium, and the like, as well asthe fluorides.

According to this embodiment, the silica particulates advantageouslyhave a number of OH⁻ /nm² (NOH) equal to or less than 10 and moreparticularly ranging from 4 to 10.

For the silicas according to this embodiment, ZCP ranges from 3 to 6.5.

The methods for determining NOH and ZCP are described above inEmbodiment I.

Moreover, in order to improve the compatibility of the silicaparticulates according to this embodiment with respect to otherconstituents and in particular fluorine, the content of divalent andhigher valency cations contained in the silica is at most equal to 1,000ppm. It is particularly desirable that the aluminum content of thesilica particulates of the invention be at most 500 ppm. Moreover, theiron content of the silica particulates of the invention isadvantageously at most 200 ppm. Preferably, the calcium content is atmost 500 ppm and more preferably at most 300 ppm.

The silicas according to this embodiment preferably also have a carboncontent of at most 50 ppm and more preferably at most 10 ppm.

Finally, the pH of the silica particulates according to this embodiment,measured according to standard NFT 45-007, is generally at most 7 andpreferably it ranges from 6 to 7.

The above characteristics provide a silica compatible with divalent andhigher valency metal cations and in particular zinc, strontium and tin.This compatibility, measured by the test described below, is at least30%, preferably at least 50% and more preferably at least 80%. Inaddition, the silicas according to this embodiment have a goodcompatibility with the fluoride anion, of at least approximately 80% andpreferably at least 90%.

In addition to the surface chemistry characteristics described above,which impart compatibility thereto, the silica particulates of theinvention also have physical properties which are perfectly suited fortheir use in a dentifrice.

Advantageously, the BET surface of the silica particulates of theinvention ranges from 40 to 600 m² /g, and more preferably from 40 to350 m² /g. Their CTAB surface typically ranges from 4 to 400 m² /g, andmore preferably from 40 to 200 m² /g.

The BET surface is determined by the BRUNAUER-EMMET-TELLER methoddescribed in the Journal of the American Chemical Society, Vol. 60, p.309 (February 1938) and according to the standard NF X11-622 (3.3).

The CTAB surface is the external surface determined by the ASTM standardD3785, but by using the adsorption of hexadecyltrimethyl ammoniumbromide (CTAB) at pH 9 and taking 35 A⁰² as the projected area of theCTAB molecule.

The silica of the invention may correspond to the three types usuallydistinguished in the dentifrice field.

Thus, the silica particles of the invention may be of the abrasive type.Same then have a BET surface of from 40 to 300 m² /g. In this case, theCTAB surface ranges from 40 to 100 m² /g.

The silica particles of the invention may also be of the thickeningtype. Their BET surface then ranges from 120 to 450 m² /g, and morepreferably from 120 to 200 m² /g. They may have a CTAB surface of from120 to 400 m² /g, and more preferably from 120 to 200 m² /g.

Finally, as a third type, the silica particles of the invention may bebifunctional. In this instance they have a BET surface of from 80 to 200m² /g. Their CTAB surface ranges from 80 to 200 m² /g.

The silica particles of the invention may also exhibit an oil uptake offrom 80 to 500 cm³ /100 g determined by the NFT standard 30-022 (March53) using dibutyl phthalate.

More precisely, such oil uptake ranges from 100 to 140 cm³ /100 g forthe abrasive silica, from 200 to 400 for the thickening silica and from100 to 300 for the bifunctionals.

The silica particulates preferably have, again vis-a-vis theirdentifrice applications, a particle size of from 1 to 10 μm.

This mean particle size is measured by Counter-Coulter.

The apparent density thereof generally ranges from 0.01 to 0.3. In apreferred embodiment of the invention, the silica particulates areprecipitated silica particulates.

Finally, the silica of the invention has a refraction index generallyfrom 1.440 to 1.465.

Process for the Preparation of Novel Silica Particulates

The process for the preparation of the silica particulates of thisembodiment of the invention will now be described in greater detail. Asindicated above, this process is of the type comprising reacting asilicate with an acid, resulting in the formation of a silica suspensionor a silica gel.

It will be appreciated that any known operation may be used to preparethis suspension or gel (addition of acid to a silicate sediment,simultaneous total or partial addition of acid and silicate to a watersediment, or silicate solution, etc.), with the selection being madeessentially as a function of the physical characteristics of the silicawhich is sought to be produced.

In a preferred embodiment, the silica gel or suspension is prepared bysimultaneously adding the silicate and the acid to a sediment, which canbe a water sediment, a colloidal silica dispersion containing 0 to 150g/l of silica, expressed as SiO₂, a silicate or an inorganic or organicsalt, preferably of alkali metals, such as, e.g., sodium sulfate orsodium acetate. The addition of these two reagents is carried outsimultaneously in such manner that the pH is maintained constant at avalue of from 4 to 10, preferably from 8.5 to 9.5. The temperatureadvantageously ranges from 60° to 95° C.

One technique for preparing the colloidal silica dispersion, preferablyhaving a concentration of from 20 to 150 g/l entails heating an aqueoussilicate solution, e.g., at a temperature of from 60° to 95° C., andadding the acid to said aqueous solution until a pH is obtained rangingfrom 8.0 to 10.0 and preferably close to 9.5.

The concentration of the aqueous silicate solution, expressed as SiO₂,preferably ranges from 20 to 150 g/l. It is possible to use a diluted orconcentrated acid, and its normality can range from 0.5 to 36N,preferably from 1 to 2N.

The silicate is advantageously an alkali metal silicate and preferably asodium silicate, with a SiO₂ /Na₂ O weight ratio of from 2 to 4 andpreferably equal to 3.5. The acid can be gaseous, such as carbon dioxidegas, or liquid, preferably sulfuric acid.

In a further stage of the process of this embodiment, the suspension orgel is subjected to a double aging operation. A first aging is carriedout at a pH of at most 8.5 and, e.g., ranging from 6 to 8.5 andpreferably at 8.0. Aging is preferably carded out hot, e.g., at atemperature of from 60° to 100° C. and preferably at 95° C. for a periodof time ranging from 10 minutes to 2 hours.

Another variant of the present embodiment comprises preparing a silicagel or a silica suspension by progressively adding the acid to asediment containing the silicate, until the desired aging pH isattained. This operation is carried out at a temperature preferablyranging from 60° to 95° C. The suspension of the silica gel is then agedunder the conditions described hereinbefore.

This is followed by a second aging at a pH below 5, preferably rangingfrom 3 to 5 and even more preferably ranging from 3.5 to 4.0. Thetemperature and time conditions are the same as for the first agingstep. Acid is added to attain the desired aging pH.

It is also possible to use an inorganic acid such as nitric,hydrochloric, sulfuric or phosphoric acid, or even carbonic acid formedby bubbling carbon dioxide gas.

The silica is then separated from the reaction medium by any knownmeans, e.g., a vacuum filter or a filter press. Thus, a silica cake isrecovered.

The next stage of the process according to this embodiment entailswashing the silica cake thus produced. Washing is carried out underconditions such that the pH of the suspension or medium prior to dryingis in accordance with the following equation:

    pH=d-e log (D)                                             (II)

in which e is a constant equal to or less than 1.0; d is a constantequal to or less than 8.5; and (D) is the electrical conductivity of theaqueous silica suspension expressed in microsiemens.cm⁻¹ (μS/cm).

The washing is with water, preferably at a temperature ranging from 40to 80° C. As a function of the particular case, one or more andgenerally two washing operations are carried out with water, preferablydeionized water and/or using an acid solution having a pH of from 2 to7. This acid solution may be, for example, a solution of an inorganicacid such as nitric acid.

However, preferably, said acid solution can also be an organic acidsolution, particularly of a complexing organic acid. This acid can beselected from among carboxylic, dicarboxylic, hydroxycarboxylic andaminocarboxylic acids.

One example of such an acid is acetic acid and examples of thecomplexing acids are tartaric, maleic, glyceric, gluconic and citricacid.

Particularly when using a solution of an inorganic acid, it can beadvantageous to carry out a final washing with deionized water.

From a practical standpoint, the washing operations can be carried outby pouring the washing solution onto the cake, or by introducing thelatter into the suspension obtained, following the crumbling of thecake. Thus, the filter cake, prior to the drying operation, is subjectedto crumbling or disintegration, which can be carried out by any knownmeans, e.g., a high speed stirrer.

Thus, before or after washing, the silica cake is comminuted and thendried by any known means. Drying can be carried out in a tunnel ormuffle furnace, for example, or by atomization in a hot air stream, theinlet temperature of which can range from approximately 200° to 500° C.and whose outlet temperature ranges from approximately 80° to 100° C.The residence time advantageously ranges from 10 seconds to 5 minutes.

If necessary, the dried material can be ground to provide the desiredgrain or particle size. This operation is carded out in a conventionalapparatus, such as an impeller mill or an air jet grinder.

Improved Dentifrice Compositions.

This embodiment of the invention also features novel dentifricecompositions containing the silicas described above, or prepared by theprocess also described above.

The composition of the dentifrice has been described above in EmbodimentI. More specifically, the improved dentifrice also includes constituentsproviding divalent and higher valency metal cations, those which aremost typically used are zinc citrate, zinc sulfate, strontium chlorideand tin fluoride.

Illustrative Examples

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

In said examples to follow, the pH measuring protocol as a function ofthe conductivity and concentration, as well as the tests for measuringthe compatibility of the silica with the various constituents, werecarried out according to the following techniques:

pH Measurement protocol as a function Of the silica concentration andits conductivity;

Silica suspensions having increasing concentrations ranging from 0% to25% by weight were formed by dispersing a mass 100 m of degassed,deionized water (Millipore quality). The suspensions were stirred for 24hours at 25° C.

The pH of the suspensions and solutions obtained after centrifuging afraction of the suspension at 8,000 r.p.m. for 40 min, and filtering ona 0.22 μm Millipore filter, were measured at 25° C. under a nitrogenatmosphere using a Titroprocessor Metrohm 672-type measuring system.

In the same manner, the conductivity of the resulting suspensions andsolutions was measured at 25° C. using a Radiometer conductivity meter(CDM83) equipped with a CDC304 cell with a cell constant equal to 1cm⁻¹. The conductivity is reported in μS/cm.

The suspension effect (SE) is defined as the difference in pH betweenthe pH of a 20% silica suspension and the pH of its supernatant solutionseparated by centrifugation.

Measurement of compatibility with tin fluoride SnF₂ :

(1) An aqueous solution containing 0.40% SnF₂ and 20% glycerine wasformed by dissolving 0.40 g of SnF₂ and 20 g of glycerine in 79.60 g oftwice distilled water.

(2) 4 g of silica were dispersed in 16 g of the solution produced in(1). The pH of the suspension was adjusted to 5 by the addition of 0.1NNaOH. The thus obtained suspension was stirred for 4 weeks at 37° C.

(3) The suspension was then centrifuged at 8,000 r.p.m. for 30 min. andthe supernatant obtained was filtered on a 0.22 μm Millipore filter.

(4) The free tin concentration was determined by atomic absorption inthe solution obtained in (1) and in the supernatant obtained in (3).

(5) The compatibility was determined by the following relationship:##EQU14## Hereinafter, the percentage tin compatibility is designatedSn. Measurement of compatibility with strontium chloride SrCl₂.6H₂ O:

(1) An aqueous solution containing 1% SrCl₂.6H₂ O was formed bydissolving 1 g of SrCl₂.6H₂ O in 99 g of twice distilled water. The pHof the suspension was adjusted to 7.0 by adding 0.1N NaOH.

(2) 4 g of silica were dispersed in 16 g of the solution obtained in(1). The thus obtained suspension was stirred for 4 weeks at 37° C.

(3) The suspension was then centrifuged at 8,000 r.p.m. for 30 min. andthe supernatant obtained was filtered on the 0.22 μm Millipore filter.

(4) The free strontium concentration was determined by atomic absorptionin the solution produced in (1) and in the supernatant produced in (3).

(5) The compatibility was determined by the following relationship:##EQU15## Hereinafter, the percent strontium compatibility is designatedSr.

The measurement of compatibility with fluorides and sodium and potassiumpyrophosphates is as described in Embodiment I.

EXAMPLE 22

Into a reactor equipped with a temperature and pH regulating system anda propeller stirring system (Mixel), were introduced 8.32 liters ofsodium silicate having silica concentration of 130 g/l and a SiO₂ /Na₂ Omolar ratio of 3.5 and 8.33 liters of soft water having a conductivityof 1 μS/cm. After beginning the stirring operation (350 r.p.m.), thethus formed sediment was heated to 90° C.

When this temperature was reached, sulfuric acid at an 80 g/lconcentration was added at a constant flow rate of 0.40 l/min to adjustthe pH to 9.5.

This was followed by the simultaneous addition of 45.25 l of sodiumsilicate at a silica concentration of 130 g/l, a SiO₂ /Na₂ O molar ratioof 3.5 and a flow rate of 0.754 l/min, as well as 29.64 l of 80 g/lsulfuric acid. The sulfuric acid flow rate was adjusted such as tomaintain the pH of the reaction medium at a constant value of 9.5.

After 60 min of addition, the sodium silicate addition was terminatedand the sulfuric acid addition was continued at a flow rate of 0.494l/min until the pH of the reaction mixture was stabilized at 8.0. Duringthis phase, the temperature of the medium was increased to 95° C. Thiswas followed by aging for a period of time of 30 min at said pH and 95°C. During aging, the pH was maintained at 8 by adding acid. Uponcompletion of the aging, the pH was adjusted to 3.5 by adding sulfuricacid and this pH value was maintained for 30 min.

After the heating was discontinued, the mixture was filtered and thefilter cake obtained was washed with 20 l of deionized water and heatedto 80° C. The cake obtained after washing was dispersed in deionizedwater to form a suspension having a silica concentration equal to 10%.

This was followed by a second filtration with water washing, such as toadjust the conductivity to 500 μS/cm. The cake was next washed withwater having a pH adjusted to 5 by citric acid, such as to adjust the pHto a value below 6. A final washing with deionized water was thencarried out.

The pH of the aqueous suspension of the disintegrated cake, having a 20%SiO₂ content, satisfied the following relationship:

    pH≦8.20-0.91 log (D)

The silica was dried by atomization. It was then ground using animpeller mill to produce a powder, the average agglomerate diameter ofwhich, measured on a Coulter counter, was 8 μm.

The physicochemical characteristics of the thus obtained silica arereported in the following table:

    ______________________________________                                        BET surface, m.sup.2 /g 65                                                    CTAB surface, m.sup.2 /g                                                                              60                                                    DOP absorption, ml/100 g of silica                                                                    125                                                   Pore volume, Hg, cm.sup.3 /g                                                                          2.1                                                   pH (5% water)           6.2                                                   Refractive index        1.450                                                 Translucency %          90                                                    SO.sub.4.sup.=  ppm     100                                                   Na.sup.+  ppm           60                                                    Al.sup.3+  ppm          150                                                   Fe.sup.3+  ppm          100                                                   Ca.sup.2+  ppm          10                                                    Cl.sup.-  ppm           20                                                    C ppm                   20                                                    ______________________________________                                    

Table IV below sets forth the surface chemistry characteristics of thesilica according to the invention and Table V the results of thecompatibility tests with the metal cations: zinc, tin, strontium, andwith the standard components of dentifrice formulations: fluoride andpyrophosphate.

EXAMPLE 23

Into a reactor equipped with a temperature and pH regulating system anda propeller stirring system (Mixel) were introduced 530 l of sodiumsilicate at a 135 g/l silica concentration and a SiO₂ /Na₂ O molar ratioof 3.5 and 15 l of soft water having a conductivity of 1 μS/cm. Afterbeginning the stirring operation (350 r.p.m.), the thus formed sedimentwas heated to 90° C. When this temperature was reached, sulfuric acid ata concentration of 80 g/l was added at a constant flow rate of 0.38l/min to adjust the pH to 9.5.

This was followed by the simultaneous addition of 44.70 l of sodiumsilicate at a silica concentration of 135 g/l, a SiO₂ /Na₂ O molar ratioof 3.5 and a flow rate of 0.745 l/min, as well as 25.30 l of 80 g/lsulfuric acid. The sulfuric acid flow rate was adjusted such as tomaintain the pH of the reaction medium at a constant value of 9.5.

After 60 min of addition, the sodium silicate addition was terminatedand the sulfuric acid addition was continued at a flow rate of 0.350l/min until the pH of the reaction mixture was stabilized at 7. Duringthis phase, the temperature of the medium was increased to 95° C. Thiswas followed by aging for 30 min at this pH and at 95° C. During aging,the pH was maintained at 7 by adding acid. Upon completion of the agingstep, the pH was adjusted to 4 by adding sulfuric acid and this pH wasmaintained for 30 min.

After discontinuing heating, the mixture was filtered and the filtercake obtained washed with deionized water until a filtrate was producedhaving a conductivity of 2,000 μS/cm.

The cake was then disintegrated in the presence of water to form a 20%silica suspension.

A final washing stage was carried out using deionized water, such thatthe pH of the aqueous suspension of the disintegrated cake having a 20%SiO₂ content satisfied the following relationship:

    pH≦8.20-0.91 log (D).

The silica was dried at 120° C. for 24 hours and then ground on animpeller mill to produce a powder, the mean agglomerate diameter ofwhich was 8 μm.

The physicochemical characteristics of the thus obtained silica arereported in the following table:

    ______________________________________                                        BET surface, m.sup.2 /g 85                                                    CTAB surface, m.sup.2 /g                                                                              80                                                    DOP absorption, ml/100 g of silica                                                                    150                                                   Pore volume, Hg, cm.sup.3 /g                                                                          3.20                                                  pH (5% water)           6.5                                                   Refractive index        1.455                                                 Translucency %          70                                                    SO.sub.4.sup.=  %       0.5                                                   Na.sup.+  %             0.05                                                  Al.sup.3+  ppm          250                                                   Fe.sup.3+  ppm          120                                                   Ca.sup.2+  ppm          50                                                    Cl.sup.-  ppm           20                                                    C ppm                   5                                                     ______________________________________                                    

The following Table IV sets forth the surface chemical characteristicsof the silicas according to the invention described in Examples 22 and23. It also reports the result of the compatibility of the silicasaccording to the invention with the metal cations zinc, tin, strontiumand with the conventional components of dentifrice formulations, namely,fluoride and pyrophosphate.

For comparison purposes, Tables IV and V also report the characteristicsand compatibilities of commercially available silicas, the followinglist constituting a representative range of standard silicas:

Si1: Syloblanc 81 (GRACE)

Z113 : Zeodent 113 (HUBER)

Sid12: Sident 12 (DEGUSSA)

Sy115: Sylox 15 (GRACE)

T73 : Tixosil 73 (RHONE-POULENC)

T83 : Tixosil 83 (RHONE-POULENC).

                  TABLE IV                                                        ______________________________________                                                 Physicochemical characteristics of silicas                           Silica     pH log (D)                                                                              SE         Ho   ZCP                                      ______________________________________                                        S81        7.0-0.62 z                                                                              -0.17      ≦2                                                                          2.2                                      Z113       10-1.0 z  -0.70      ≦3                                                                          2.5                                      Sid12      8.5-0.60 z                                                                              -0.20      ≦3                                                                          2.8                                      Sy115      9.2-0.74 z                                                                              -0.94      ≦3                                                                          2.5                                      T73         10-0.87 z                                                                              -0.20      ≦3                                                                          3.0                                      T83        8.6-0.60 z                                                                              -0.18      ≦3                                                                          2.5                                      Example 22 8.0-0.50 z                                                                              -0.00      ≦4                                                                          4.2                                      Example 23 7.4-0.30 z                                                                              -0.03      ≦4                                                                          4.0                                      ______________________________________                                    

The definitions of the symbols used in the above table are given below:

pH/log (D) represents the equation pH=b-a log (D), in which b and a aretwo constants and D is the conductivity of the silica suspension inμS/cm;

SE represents the suspension effect measured by the relation SE=pHsuspension-pH supernatant defined above;

Ho is the Hammett constant;

ZCP represents the pH at which the surface charge of the silica is zero.

                  TABLE V                                                         ______________________________________                                        Compatibilities of silicas with active molecules:                                     % Compatibilities                                                     Silica    Zn        Sn    Sr      F   P.sub.2 O.sub.7                         ______________________________________                                        S81       0         25    20      90  80                                      Z113      0         15    10      95  90                                      Sid12     10        25    20      90  80                                      Sy115     0         10    10      90  80                                      T73       20        15    10      90  90                                      R83       10        10    10      95  95                                      Example 22                                                                              80        60    90      95  95                                      Example 23                                                                              85        75    95      95  90                                      ______________________________________                                    

The silicas according to this embodiment markedly differ fromconventional silicas as a result of their physicochemicalcharacteristics and their good compatibility with zinc, tin andstrontium.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. A process for the preparation of silicaparticulates adapted for formulation into dentifrice compositions,having a surface chemistry as to be at least 65% compatible withguanidine values, comprising reacting a silicate with an acid to form asuspension or gel of silica, separating such silica suspension or gel,first washing said separated silica with water until the conductivity ofthe filtrate is at most 2,000 microsiemens.cm⁻¹, and then again washingsaid separated silica, either with water or an acid solution.
 2. Theprocess as defined by claim 1, comprising again washing said separatedsilica with a solution of an organic acid.
 3. The process as defined byclaim 2, said organic acid comprising a carboxylic, dicarboxylic,aminocarboxylic or hydroxycarboxylic acid.
 4. The process as defined byclaim 3, said organic acid comprising acetic, gluconic, tartaric,citric, maleic or glyceric acid.
 5. The process as defined by claim 1,comprising aging the suspension or gel of silica prior to the separationthereof.
 6. The process as defined by claim 1, comprising adding analkaline earth metal salt to said suspension or gel of silica, or tosaid separated silica.